The influence of annealing and thermal history on the microhardness, MH, of three vinyl alcohol-ethylene copolymers has been studied and the results compared with those for the corresponding homopolymers, poly(vinyl alcohol) and polyethylene. The results show that the microhardness of the copolymers increases with the vinyl alcohol content, although the degree of heterogeneity also seems to play an important role. Two thermal histories (fast quenching or slow cooling from the melt) have been imposed on the samples. The quenched (Q) specimens present lower values of MH than the corresponding slowly cooled (S) samples. Both Q and S samples were subsequently annealed at different temperatures. In the annealed samples, it has been found that the variation of MH is different for the two thermal histories. Thus, the Q samples show an increase of MH with the annealing temperature while the contrary is found for the S specimens. These results can be explained by the heterogeneous nature of these copolymers, as revealed by differential scanning calorimetry traces, which show an appreciable degree of material melting at temperatures well below the main melting peak.