Two tunable dye lasers have been used to generate coherent radiation at a tunable sum frequency in the vacuum-ultraviolet spectral region. Spectral ranges of 3500 cm-1 around 1895 and 1200 cm-1 around 1798 have been covered. These ranges may easily be extended. Large (×104) enhancements of 21+2 sum mixing are seen when 21 is tuned to a double-quantum allowed transition from the ground state in Sr vapor. © 1974 The American Physical Society.