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In the event of a corporate spin-off, how can I calculate the correct cost basis for each company's shares?
From my understanding: Original Holding:  Siemens - 10,000 units at 80 Euros/unit Cost = 800,000 euros Spin-off: Every 10 Siemens get 1 OSRAM On July 5th, 2013: Siemens closing - 78 Euros On Monday, July 8th: Ex-date (opening) - 75 Euros Hence: Market value for:- 1.  Siemens: 75 * 10,000 = 750,000 euros 2.  OSRAM: (10,000 / 10) * (78 - 75) = 3,000 euros Total Market value = 780,000 + 3,000 = 753,000 euros Ratio for: 1. Siemens = 750,000 / 753,000 = 0.996015936 2. OSRAM = 3,000 / 753,000 = 0.003984063 Cost for: 1. Siemens = 800,000 * 0.996015936 = 796,812.75 2. OSRAM = 3,187.25