Significant enhancement of the unsaturated conversion efficiencies of four‐wave sum frequency mixing processes involving autoionizing states in Sr vapor is obtained by utilizing the 5s7s 1S0two‐photon channel and phase matching by frequency adjustment. Extrapolation to low power levels indicates that production of cw vuv radiation may be possible. A saturation effect arising from depletion of one of the fundamental beams by two‐photon absorption is observed.