Lactic acid is converted to 2,3-pentanedione, acrylic acid, and acetaldehyde in vapor-phase reactions at 0.5 MPa over phosphate salts. Highest selectivities to 2,3-pentanedione are achieved at 280-300-degrees-C and long (2-4 sec) residence times, while selectivity to acrylic acid is best at 350-degrees-C and short residence times (0.4 sec). 2,3-Pentanedione is proposed to form by a second-order condensation in the presence of phosphate, while acrylic acid is proposed to result from elimination of water via a cyclic lactic acid-phosphate transition state.