It was considered a tax free spin off meaning no tax was due at the time of the spin. Not all such transactions are tax free. If you own both COP and PSX in a non-IRA or other tax deferred account, you will need to pay capital gains tax when you decide to sell them.. Therefore, you need to know the price each company was at the time of the split. That ratio can then be applied to your original purchase price in order to calculate your cost basis. The method is probably available on COP's web site.
Thanks for the info .... I think what I will do is sell both at the same time and just figure out the gains using the original cost of COP and then add COP and PSX together when I sell and get my total gain. Does that sound right to you?