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Microhardness (MH) data as a function of temperature for two amorphous polymers [poly(methylmethacrylate) and poly(vinylacetate)] and two semicrystalline polymers [poly(ethyleneterephthalate) and poly(arylether ether ketone)] quenched into the amorphous state are presented, tt is shown that MH can conveniently detect the glass transition temperature (Tg) for the above mentioned polymers. Molecular rearrangements taking place above and below Tg, such as physical ageing leading to a more compact molecular packing, and thermal expansion can also be followed by means of MH measurements. Finally, the presence of a crystalline phase in these materials has been shown to shift the Tg value towards higher temperatures.