A novel process for the amidation or esterification which comprises reacting a compound having a carboxy group with a compound having an amino or imino group which can be acylated or with a compound having a hydroxy group in the presence of a sulfonic acid ester of the formula: R.sub.1 --SO.sub.2 --OR.sub.2 wherein R.sub.1 is an organic group and R.sub.2 O-- is a residue of a strongly acidic N-hydroxy compound as a condensation agent, and a novel sulfonic acid ester useful as such a condensation agent and a process for the preparation thereof.