Experiments to verify a semiclassical model of Rydberg atom collision processes revealed discrepancies between the model predictions and experimental results for the reaction$$\rm K\sp{}(np)+SF\sb6\to K\sp{+}+SF\sbsp{6}{-*}$$at low to intermediate values of the principal quantum number n (n <≈ 20). Initial measurements using non-velocity selected Rydberg atoms yielded ion angular distributions that did not agree with model calculations. To explore this further, experiments were initiated using velocity selected Rydberg atoms. These data show that the discrepancy between the model and experiment is due to transfer of energy from the SF\sbsp6−∗ ion into translational energy of the product ion pair through a close collision. This process is not included in the semiclassical model. This conclusion is substantiated by study of the dissociative electron transfer reaction$$\rm K\sp{}(np)+CH\sb3I\to K\sp{+}+CH\sb3I\sp{-*}\to K\sp{+}+I\sp-+CH\sb3$$for which similar energy transfer cannot occur. The data agree well with model calculations.