Conductivity, thermally stimulated depolarisation (tsdc) and thermal studies are carried out on pure and Cu2+-doped potassium hydrogen maleate single crystals. Bothdta anddsc reveal first-order phase transition at 503 K.dsc further shows that the crystal decomposes even before the transition is complete. A ‘knee’ is observed around 505 K in conductivity plots at this transition. Thetsdc of Cu2+-doped system gave a single peak at 241 K and the reasons are analysed. Activation energies from conductivity andtsdc suggest proton conduction. However no direct evidence in coulometric experiment could be observed. But thermoelectric power measurements reveal the charge carriers to be positive.