An alcohol solution of phenyl ketone C(6)H(5)COR(1), where R(1) is CH3, C2H5, H, Or C6H5, was introduced into a vacuum as a continuous liquid flow (liquid beam) and irradiated with a laser beam at wavelengths of 250, 280, and 355 nm. Ions produced by multiphoton ionization in the liquid beam and ejected from it were analyzed by time-of-flight mass spectrometry. The mass spectra of the ions produced from various alcohol solutions of phenyl ketones at different excitation wavelengths indicate that a protonated phenyl ketone ion, C6H5C(OH)R(I)(+), produced by laser irradiation reacts with an alcohol molecule, R(2)OH, in the solution and C6H5C(OR(2))R(I)(+) is produced. This reaction corresponds to the initial process for acetal formation from a phenyl ketone and an alcohol, and these product ions are identified to be its reaction intermediates. When an alcohol solution of benzophenone, C6H5COC6H5, was used, a pinacol ion, (C6H5)(2)C(OH)C(OH)(C6H5)(2)(+) was produced in addition to C6H5C(OR(2))C6H5+. The appearance of the pinacol ion suggests the presence of benzophenone dimers in the vicinity of the liquid surface.