The azo coupling reaction of 2-naphthol with diazotized 2-(4-aminophenylsulfonyl)ethyl groups bound to macroporous bead cellulose was studied. The reaction was followed spectrophotometrically by determining 2-naphthol in the reaction mixture. The amount of 2-naphthol bound to the sorbent decreases with increasing pH in the pH range 5-9 as a result of simultaneous influence of the sulfonyl groups on the stability of the diazonium salt and limited mobility of the diazonium groups bound to the sorbent. Approximately a 1.3 molar excess of 2-naphthol and a 1-h reaction time at 25-degrees-C are sufficient for a maximum reaction yield at a given pH. The method makes possible the semiquantitative analysis of the amount of amino groups in the sorbent which are accessible to coupling.