knowledge of molt and plumage patterns for neotropical birds is incipient. taking into account its usefulness to determine age and sex reliably in birds, i present data for some morphometric variables and external characters of mionectes olivaceus (tyrannidae), evaluate its ability to differentiate among age and sex and suggest a fast and reliable method for sex and age identification. i measured 106 museum specimens of four subspecies, among which i found no morphometric differences. i found important differences between sexes and ages. though rectrix length and wing chord did vary between sexes, as has been reported for other members of the genus mionectes, they did not contribute to group formation in three discriminant function analyses. beak depth and two measures of the ninth primary emargination were the most relevant variables for ageing and sexing. these variables allowed the analysis to correctly classify more than 75 % of all the cases evaluated. long and deep emarginations, deep beaks and truncated external rectrices are key characters to age determination. beak depth is the key character to differentiate among sexes, along with length and depth of the ninth primary emargination. results from this kind of studies provide invaluable tools for an accurate and fast identification, which are essential requirements in demographic and ecological studies or in banding and monitoring programs.