Dehydration of 1,2-ethanediol and 2-aminoethanol and dehydrohalogenation of 2-chloro- and 2-fluoroethanol over H-ZSM-5 at 200-350 degrees C was followed under low-pressure (ca. 25 mTorr) flow conditions using ultraviolet photoelectron spectroscopy as a direct on-line monitor. This circumvents the necessity of separations since the effluent from the reactor can be monitored immediately after leaving the catalyst bed. Under these conditions all precursors are consumed, leading to the final product acetaldehyde, except in the case of 2-aminoethanol where dehydration gives, in high yield, the unexpected N-methyl isomer of methylenimine, an unstable molecule. A postulated mechanism for this result, applicable also to the formation of acetaldehyde in the other beta-substituted alcohols, suggests the intermediacy of three-membered cyclic species, namely, ethylenimine and ethylene oxide or their protonated analogs.