Under the conditions of chemical ionization (NH3, isobutane, ND3), hydroxyacetals do not show the expected [M + H+] signal, but the peak corresponding to the loss of the alcohol component of the acetal. This reaction is dependent on the presence of the OH group in the vicinity of the acetal and on the ring size of the heterocycle being formed. M+· is detected to a small extent as a consequence of charge exchange during chemical ionization.