General procedures are outlined for the detection of the presence of interferences during spectrophotometric analysis using first-derivative absorption spectra. These involve (i) ratios of first-derivative maxima, which are independent of concentration, and (ii) the compensated derivative curves. Thus, selecting two suitable maxima at λ1 and λ2, D11/D12, |D11–D12|/D11 and D1(corr.)1/|D11–D12| are used to reveal the shape of the interference curve and the possibility of eliminating its effect. These ratios are highly reproducible with coefficients of variation of less than 1%. The methods are illustrated by the determination of phenol in a pharmaceutical preparation. The results obtained were in good agreement with those of the official bromimetric method.