A longitudinal event‐history analysis using complete data on the aircraft fleets of all British airlines over the period 1919 to 1975 was applied to elucidating how a firm's internal resource demography can explain its resource divestiture decisions. The likelihood of divestiture for any particular aircraft was found to depend strongly on its age and size, but also on the age and size heterogeneity of an airline's entire fleet. The results constitute a demonstration of the utility of taking a demographic approach to studying the management of resource portfolios. This approach may also prove fruitful in the study of other types of strategic resources and their management.