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.
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.
The anti-takeover plan it adopted gives shareholders a preferred stock-purchase right for each share they own at the close of business Aug. 5. If the rights become exercisable, shareholders can pay the strike price to get shares with a market value of twice that price.
The plan would kick in if an investor or group acquires 10 per cent of the company's stock — 20 per cent for institutional investors — before July 24, 2014.
Air Products was thwarted 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 the business strategy of nutrition supplement company Herbalife.
Shares of Air Products reached an all-time high price of $109.90 Wednesday morning before retreating to $108.65. The stock was still up nearly 3 per cent, or $3.12, to $108.73 in late-morning trading.
The stock has climbed almost 30 per cent this month alone.