This paper discusses the application of thermomicroscopy in the temperature range 25-250 degreesC for the characterisation of 100- and 200-year-old artists' paint. Studies of 20th century reference materials, and paint samples from Sir Joshua Reynolds (1723-1792), J.M.W. Turner (1775-1851) and J.A.M. Whistler (1834-1903) are discussed, to illustrate the utility of thermomicroscopy both for the materials analyst and for the paintings conservator. Turner's bituminous paint is prone to darken irreversibly at 75-88 degreesC, while the 'megilp' formulations which he used are not distinguishable from oil paint with lead driers or oil with some resin added, though the group are distinguishable from unmodified oil medium. Beeswax and spermaceti wax melt out from Turner's paint formulations at the typical melting point for the wax, whereas Reynolds' paint formulations including beeswax have aged in a more complex way, and show melting point ranges which lie within those of the original components. Whistler's paints tends to darken irreversibly at 70-90 degreesC due to the presence of additives in his manufactured tube paints, and their measured melting points indicate a complex ageing process which may be analogous to that in Reynolds' paint formulations.