BP’s chairman is nursing a paper loss of almost £2m ($3.1m) after the spill in the Gulf of Mexico in April.
Carl-Henric Svanberg, who joined the UK oil group in January, owns 925,000 shares which he bought in two separate lots.
He bought 750,000 shares on February 3 at £5.75 a share.
BP’s share price had risen to £6.55 before the explosion of the Deepwater Horizon rig in the gulf on April 20.
Eight days after the accident the shares had dropped to £6.19 and Mr Svanberg bought an additional 175,000 shares on April 28.
At the time, the full implications of the disaster had yet to be digested by the market.
The explosion killed 11 workers and has led to the worst environmental disaster in US waters, spewing 4.9m barrels of oil into the gulf.
BP’s shares collapsed a few weeks later, dropping to as low as 296p during intraday trading in London on June 29.
The company has since managed to stem the leak.
It is still drilling a relief well to intersect with the ruptured Macondo well and then pump cement in through that to seal it from the bottom.
BP’s shares closed on Thursday in London at 385.60p, valuing Mr Svanberg’s holding at £3.56m compared with the £5.4m he paid for them, a drop of £1.8m.
Tony Hayward, BP’s outgoing chief executive who owns more than 587,000 shares, and other BP executives have also seen the value of their shareholdings drop substantially.
Despite the decline in value of his BP shares, Mr Svanberg remains one of Sweden’s wealthiest businessmen after six years as chief executive of Ericsson, the world’s largest network equipment maker, until his retirement last year.
His appointment at BP – eight times the size of Ericsson in terms of revenue – was a surprise because of his relatively low profile outside Sweden and his lack of experience in the oil industry.