The latest deal, primarily funded by a $6 billion loan, will combine the operators of Thailand's biggest convenience stores and cash-and-carry businesses, giving CP All greater bargaining power in sourcing supplies and the muscle to expand in Southeast Asia.
"This is a format that has been very successful in Thailand and could be rolled out to Southeast Asia more generally," said David Chin, co-head of investment banking in Asia for UBS, which was among the lead arrangers for CP All for financing of the Siam Makro transaction.
Thai companies have been on an acquisition binge in the last two years, encouraged by cheap bank debt, rising cash piles and surging share prices. That took Thai M&A volume to a record $25.9 billion last year.
CP All holds more cash than all but one Southeast Asian retailer, according to Thomson Reuters data, with $1.15 billion in cash and equivalents, just behind SM Investments, which has $1.8 billion. The world's third-largest operator of 7-Eleven stores, CP All aims to have 10,000 of the outlets in Thailand by 2018.
Dhanin and SHV founded Siam Makro in 1988, and by 1997 Dhanin's Charoen Pokpphand group was its biggest shareholder. The crash of the Thai baht in 1997 forced Dhanin to sell holdings including Siam Makro and Lotus Supercenter, which was acquired by British retailer Tesco.
The offer values Siam Makro at 53 times historic price-to-earnings, making it the most expensive retail stock in the Asia-Pacific region, according to Thomson Reuters data.
This is the second debt-backed deal by companies linked to Dhanin in the last three months. The Ping An stake buy was part-funded with a $5.5 billion loan from UBS, Reuters previously reported.
The Siam Makro deal will be funded by a $6 billion loan arranged by HSBC, Siam Commercial Bank, Standard Chartered, Japan's Sumitomo Mitsui Banking Corp., and UBS, two people with direct knowledge of the matter said, speaking on condition of anonymity because financing details are not public.
CP All, which will be the borrower, said it does not plan to issue new shares.
HSBC was the sole advisor to SHV Holdings, while Siam Commercial Bank advised CP All, the people familiar said.
Other companies that earlier showed interest in Siam Makro included Berli Jucker, a trading firm controlled by beer tycoon Charoen Sirivadhanabhakdi and Central Group, Thai media reports previously said.
"CP All is the only bidder to offer the price. It seems like the deal was done before other bidders joined the bid," said a source with direct knowledge of the offer, speaking on condition of anonymity as the deal was confidential.
CP All's offer represents a 15.4 percent premium to Siam Makro's last traded price on Friday, before its shares were halted on Monday pending an announcement.
CP All's $6.6 billion offer for Siam Makro would be the biggest retail M&A in the world this year, and double the size of the No. 2 deal, according to Thomson Reuters data.
Siam Makro, controlled by privately held Dutch trading house SHV Holdings, has 58 Makro-branded outlets in Thailand, mainly selling food in bulk to hotels, restaurants and smaller retail outlets. It made a 2012 net profit of 3.56 billion baht ($124.11 million), up 36 percent year-on-year, but it has been the country's slowest-expanding retailer as a result of stricter rules on large stores.
Competition for Thai shoppers' business has intensified since the Chirathiwat family, which owns the country's largest retailer Central Group, bought a stake in the local unit of Japanese-based Family Mart last year.
Lawson Inc., Japan's second-largest convenience store chain, has also formed a joint venture with Saha Pattanapibul, part of the Saha Group, Thailand's leading maker and distributor of consumer products.
Earlier on Tuesday CP All's shares were suspended pending an announcement.