The numerical balance between males and females in a population has been of long-standing interest to social scientists because of its effects on marriage, fertility and the societal roles of men and women. Yet little attention has been paid to mapping changes in sex ratios or identifying their demographic determinants. This study addresses this gap by examining secular changes in the balance between males and females in Australia from 1901 to 2008. A method of decomposing these changes into components due to sex differentials in fertility, mortality and migration is described and applied. The study describes the gradual feminisation of the Australian population over a century, and gives an account of how the ratio of the sexes differs by age and how long it takes to exhaust the numerical superiority of males at birth. The study explains these changes by examining long-term trends in the sex ratio of births, deaths and net migration for successive cohorts. The relatively high mortality of males, and the emerging pattern of feminine net migration since the 1970s, explain the transformation of the Australian population from its highly masculine past to its contemporary state of greater balance between the sexes.