Thermoelectric power and electrical conductivity of pure and 5, 10 and 20% strontium-doped lanthanum manganite are determined as function of temperature in air and of P-O2 at 1000 degrees C. At high temperatures the thermoelectric power is negative. Both thermoelectric power and conductivity are found to be nearly independent of doping at high temperatures, but not at low. This can be explained if it is assumed that Mn3+ disproportionates into Mn2+ and Mn4+. In reducing atmospheres the thermoelectric power is found to be only slightly dependent on P-O2, which also might be explained by the disproportionation reaction.