Sensitive and robust monitoring of product and reactants in a complex bioconversion stream is essential for the development of effective process-control strategies. In this contribution we report the use of near-infrared spectroscopy (at-line and on-line) to monitor a microbially catalysed Baeyer-Villiger bioconversion of a cyclic ketone to an optically pure lactone. The cyclohexanone monooxygenase-catalysed reaction is characterised by substrate (ketone) and product (lactone) inhibition of enzyme activity at relatively low concentrations. Quantitative multivariate calibration of a near-IR spectrophotometer for ketone and lactone resulted in a standard error of prediction at-line of 0.088 and 0.110 g/L and on-line of 0.130 and 0.180 g/L, respectively. The concentrations of substrate and product could be simultaneously monitored by near-IR, which had a response time of 5.0 and 0.75 min at-line and on-line, respectively. This work has indicated that near-IR spectroscopy has the potential to permit the realisation of an improved control strategy for this conversion based on these response times.