Rate coefficients for Hornbeck-Molnar ionisation in a K-Rb mixture at thermal energy are measured using CW laser excitation of the atoms. Special attention is paid to the formation of the KRb⁺ molecular ion in Rb(nl)-K(4S) and K(nl)-Rb(5S) collisions. The absolute rate constant values are deduced from the analysis of collisional currents by mass spectrometry techniques. They are compared with the theoretical values calculated using a model recently developed by Janev and Mihajlov (1980).