B/E Aerospace was tracking for its best day since May 2009, when it gained 17.8 percent.
Rockwell saw its worst day since August 2011, when it lost 7.2 percent.
The companies announced the acquisition Sunday, in a deal worth $6.4 billion, plus the assumption of $1.9 billion in debt. The combination expands the range of products Rockwell Collins supplies to major commercial and business aircraft.
Share price of B/E Aerospace and Rockwell Collins, year to date.
The U.S. Aerospace & Defense ETF (ITA) climbed 1 percent Monday, its best day since September 22, when the ETF gained 1.25 percent.
B/E Aerospace was leading the way for the ETF, while Rockwell Collins was dragging it down as the worst performer in the index Monday morning.
Both companies have little product overlap. Rockwell is best known for avionics, flight control systems and cabin connectivity, while B/E Aerospace is a major provider of aircraft seats, galleys, lighting and other systems.
Rockwell agreed to pay $34.10 per share in cash and $27.90 in shares of Rockwell Collins stock, a 22.5 percent premium to B/E Aerospace's closing price on Friday.
—CNBC's Gina Francolla and Reuters contributed to this report.