I bought 250 shares of company A in 2008, in 2012 it spun off company B and I received 125 shares in this new company. In 2013 I sold the 125 shares in company B but I still own the original 250 shares in company A. How do I work out the capital gains tax due. My thinking was that the cost of the shares in company B were zero as I did not buy them they were spun off, so the cgt is the total value of the shares when sold. However from searching on the internet, the cost of the shares in company B is the difference in the stock price in company A on the day before the spin off and the day after the spin off. If company A was $70 on the day before the spin off and the fell to $55 on the day after the spin off then the cost of company B is $15 x 250/150. is this correct in irish system