Both companies said Thursday their boards had formed special committees to explore a combination, though an agreement is not a given. The deal is likely to be structured as CBS, with a market value of $22 billion, buying Viacom, with a market value of $13 billion.
The sources said the Viacom transaction is seen as CBS' only option, and the two sides hope to conclude talks this quarter.
A merger of the companies, which were split apart more than a decade ago by the media mogul Sumner Redstone, has been orchestrated by his daughter, Shari Redstone. It would combine CBS's broadcast network with Viacom's cable channels and film studios as the industry faces a big shakeup.
The two held prior talks to combine but ended them in late 2016 without an agreement.