1. Heats of adsorption of acrolein, ethanol, allyl alcohol, and acetaldehyde on magnesium oxide were determined microcalorimetrically. 2. The heat of adsorption does not depend on the degree of coverage up to 60–70% of monolayer. 3. At the coverage higher than 60–70% of monolayer, interaction between the adsorbed molecules starts to manifest itself, whereby the heat of adsorption increases. 4. It is postulated that double bonds participate in the adsorption of acrolein and allyl alcohol.