In waters formed before ~1975, Chlorofluorocarbon (CFC)-12 ventilation ages are usually older than CFC-11 ages, which are in turn older than CFC ratio ages. A simple model shows that these discrepancies result from the effects of mixing on CFC ages, which are biased due to the nonlinear increases in atmospheric CFCs before 1975. CFC-11 ages are biased younger than CFC-12 ages because CFC-11's relative increase rate in the atmosphere was higher. In the model results, ideal ages are consistently older than CFC ages in waters ventilated prior to ~1975. The differences between CFC ages and ideal ages are reflected in the discrepancies among CFC-11, CFC-12, and CFC ratio ages, however, and increase with increasing importance of diffusive versus adjective tracer transport. Thus it may prove possible to estimate the magnitude of CFC age mixing biases based on differences measured between CFC-11, CFC-12, and CFC ratio ages. ¿ 2001 American Geophysical Union