The present paper deals with changes over time in the sex composition of a given population. Such changes are generated by the components of population change: natality, mortality, immigration and emigration. The objective of this paper is to propose a model which provides a quantitative statement concerning the effect of each such component on the change in the sex composition of the population. It is shown that the model is applicable to the measurement of sex composition by either the masculinity proportion or the sex ratio. To demonstrate the application of the proposed model, an example based on Canadian data is given.