This excerpt taken from the BP 6-K filed Mar 13, 2006.
Own equity instruments
The Groups holding in its own equity instruments, including shares held by Employee Share Ownership Plans (ESOPs), are classified as treasury shares, and shown as deductions from shareholders equity at cost. Consideration received for the sale of such shares is also recognized in equity, with any difference between the proceeds from sale and the original cost being taken to revenue reserves. No gain or loss is recognized in the performance statements on the purchase, sale, issue or cancellation of equity shares.