A fibre-optic sensor for p-aminophenol (PAP) based on the use of 25,26,27,28-tetrahydroxycalixarene (CAL4) immobilised onto Amberlite XAD-16 and reflectance spectrometry has been developed. The sensor is based on the reaction of PAP with CAL4 in presence of an oxidant to produce an indophenol dye. The reflectance measurements were carried out at a wavelength of 620 nm since it yielded the largest divergence different in reflectance spectra before and after reaction with the analyte. A highly linear calibration curve (r(2) = 0.992) was observed in the PAP concentration range of 0.5-35 ppm (linear within 0.5-5.5 ppm) with a calculated limit of detection (LOD), defined as the lowest concentration level of analyte that the proposed method can detect to be statistically different from an analytical blank (i.e., yielding an analytical response greater than three times the standard deviation of a blank), of 0.109 ppm. The sensor response from different probes (n = 5) gave an RSD of 1.8% at 10.9 ppm PAP concentration. The response time of the optical one-shot sensor was 5 min for a stirred solution. The intra- and inter-assay variability (as relative standard deviation: RSD) was approximately 2 and 4%, respectively. The lifetime of the blank sensor (CAL4 adsorbed onto XAD-16) was at the order of weeks-months when protected from light, while of the analyte sensor (XAD-16 + CAL4, to which the oxidation product of PAP is coupled) was at the order of days when protected from air and light. Using the optical sensing probe, PAP in urine, pharmaceutical and pharmaceutical wastewater was determined. Although aminophenols showed serious interference, the method was not affected from common ions and from various important drug active constituents present in urine and pharmaceutical effluents such as acetaminophen, acetylsalicylic acid, sorbitol, and caffeine. The proposed technique was tested by replicate analysis of several complex samples with spiked PAP, with recoveries ranging between 97 and 102%. (C) 2008 Elsevier B.V. All rights reserved.