Abstract The influence of temperature on the position of crystal-field peaks of RbMnF 3 and KMnF 3 has been investigated in the 14–550 K range. Upon warming in the 200–550 K range, the 4 T 1 g ( G) peak dependent on 10 Dq experiences a total blue shift of 470 cm -1. It is shown that only 40% of this shift arises from thermal expansion effects, the rest being due to the explicit (∂ E/∂ T) v term. The positive sign of this term is associated to higher vibration frequencies for the 4 T 1 g ( G) state (belonging mainly to the t 4 e configuration) than for 6 A 1 g ( S) as a result of smaller Mn 2+-F - distances for that excited state. Below 200 K the present data reflect the existence of magnetic and structural (only for KMnF 3) phase transitions in the compounds.