The effect of mode coupling has been calculated for radio waves partially reflected from the lower ionosphere at vertical incidence. Calculations were made for parameters appropriate to the partial reflection experiments [Belrose, 1970]. The four reflection coefficients (two direct and two coupled) are calculated for a plane discontinuity using a matrix perturbation analysis. The ratios of coupled to direct reflection coefficients are calculated for a general one-dimensional reflecting layer using the coupled equations of Clemmow and Heading . At the highest electron density considered, 104 cm−3, about 10% of the reflected extraordinary wave amplitude might be due to coupling from the incident ordinary wave. The effect is smaller at lower electron density.