The gas-phase lactic acid conversion to acetaldehyde was studied on zeolites HMCM22, HZSM5 and NaZSM5 in a plug-flow fixed-bed reactor at 583 K. HMCM22 was a strong Bronsted acid zeolite that yielded 39.7% of lactic acid decarbonylation products (acetaldehyde + CO), but formed significant amounts of coke because of lactic acid oligomerization in the narrow sinusoidal channels of the zeolite. HZSM5 exhibited a high density of Bronsted acid sites (Bronsted/Lewis acid sites ratio of 3.1) and promoted at high rates the lactic acid conversion, mainly to decarbonylation products (acetaldehyde + CO yield = 57.0%) and lactic acid oligomers. NaZSM5 contained mainly Lewis acid sites of intermediate strength (Bronsted/Lewis acid sites ratio of 0.4) that promoted the high-selectivity synthesis of acetaldehyde from lactic acid (acetaldehyde + CO yield = 96.0%) by suppressing the competitive lactic acid oligomerization and coke precursor formation. (C) 2017 Elsevier B.V. All rights reserved.