Carbonic acid esters of the formula substituent(s) substituents(s)R'.sub.1 OCOOR'.sub.2wherein R'.sub.1 is lower alkyl which may have substituent)s) selected from the group of halogen, lower alkoxy and aryloxy, or ar(lower)-alkyl which may have substituents)s) selected from the group of lower alkoxy, halogen, nitro and cyano, andR'.sub.2 is benzotriazolyl which may have halogen; or a group represented by the formula: ##STR1## wherein Y' and Z' are each cyano, nitro, carbamoyl, esterified carboxy, lower alkanoyl, aroyl or disubstituted carbamoyl; provided that whenR'.sub.2 is a group represented by the formula: ##STR2## wherein Y' and Z' are each cyano, nitro, carbamoyl or esterified carboxy, R'.sub.1 is ar(lower) alkyl having substituent(s) selected from the group of lower alkoxy, halogen, nitro and cyano.A process for the protection of amino and/or imino groups in compounds containing them by reacting them with the aforementioned esters is also disclosed.