The presence or absence of juvenile greater coverts and the shape of tail feathers were not found to be reliable ageing characters in a population of Marsh Tits in which juveniles usually replaced all the greater coverts and occasionally all of the tail feathers. Two additional characters, iris colour and primary covert shape, were found to be useful for ageing, and were checked with known-age Marsh Tits (both captive and wild) and with known-age individuals of other tit species. Sexual dimorphism is marked for wing-length and bill-length, though the former is most useful for sexing as it shows less seasonal variation. Using wing-length 63.5 mm = male and 62 mm = female gives 95% accuracy for sexing full-grown Marsh Tits throughout the year in the study population. There is some evidence for limited sexual dimorphism of bib size in Marsh Tits.