-time high Wednesday after Bill Ackman's Pershing Square revealed that it had taken nearly a 10 percent stake in the industrial gas supplier.
Just last week, Air Products put into place a poison-pill plan to foil any takeover attempts after noticing unusual activity in its stock.
A Securities and Exchange Commission filing Wednesday shows that the activist investor owns about 20.5 million shares of the company, which had more than 209.6 million shares outstanding as of June 30.
It was the largest investment ever by Ackman and it arrived the same month that he told investors in a letter that he was raising money to target a large U.S. corporation.
Air Products hit an all-time high price of $109.98 on Wednesday. It closed at $108.64, up $3.03 or 2.9 percent.
Shares of Air Products had moved at a glacial pace over the past several years compared with the Standard and Poor's 500 index. They started to take off about a month ago, rising 30 percent. The company, with a market capitalization of nearly $23 billion, took action suspecting that it was being targeted.
The poison pill allows existing shareholders to acquire more stock at a deep discount if another entity gobbles up 10 percent or more of the company.
Air Products said Wednesday that it had not been contacted by Pershing Square Capital Management, but that it "welcomes new investors and looks forward to engaging with Pershing Square to understand its views."
Air Products was thwarted by a poison pill plan in its own takeover bid of a smaller rival, Airgas Inc., more than two years ago. It abandoned that bid in early 2011.
Pershing Square also holds a sizeable stake in beleaguered retailer J.C. Penney, where Ackman had pushed for a CEO change in the summer of 2011. That led to a risky turnaround strategy that backfired and resulted in massive losses and steep sales declines.
The company's board ousted CEO Ron Johnson earlier this year after only 17 months on the job and rehired his predecessor, Mike Ullman.
Ackman also has engaged in a public feud with rival investor Carl Icahn over Herbalife, which he describes as a pyramid scheme.
Air Products and Chemicals Inc., based in Allentown, Pa., supplies gas like hydrogen and helium and equipment and services to a variety of industrial customers.