The detection of trace compounds in water using photoacoustic or photothermal techniques is presented, with specific reference to on-line water quality monitoring. A key requirement was the use of simple, robust and inexpensive light sources, whose continuous-wave output was modulated electronically at relatively low frequencies (~100Hz). Two closed-cell, low-frequency optical absorption detectors have been developed and evaluated. In both cases, a novel theoretical treatment revealed that the optimum cell design had a pressure sensor whose mechanical compliance was equal to the bulk compliance of the enclosed water. High sensitivity was achieved despite the use of lower power light sources, potentially extending the applicability of photothermal/photoacoustic methods to low-cost systems. The photoacoustic cell incorporated a thin-walled piezoelectric cylinder. Using a 678nm laser diode emitting 1.4±0.2 mW rms, a minimum detectable absorption coefficient of 3x10⁻³cm⁻¹ was demonstrated. Better performance was achieved using the photothermal detector. Photothermal expansion in aqueous samples caused the deflection of a water meniscus held across a 200 μm-radius pinhole. Meniscus displacement was monitored using fibre optic interferometry. The magnitude and form of photothermal meniscus deflection signals were shown to be in good agreement with theory. Absorption by 1.5ppb anthracene in water was detected using a mercury discharge lamp (254nm) as an excitation source, and light emitting diodes were used to excite photothermal signals in a liquid (water) for the first time. The technique was limited by light source emission intensities and environmental acoustic noise, for example giving a minimum detectable absorption coefficient of 2x10⁻⁴cm⁻¹ using an LED emitting 0.38±0.06 mW rms. The minimum detectable absorption coefficients of the photothermal detector and of a conventional transmission spectrometer were found to be similar, but the photothermal technique was far less sensitive to the effects of light scattering. The ability to detect trace contaminants in water, in the presence of turbidity, is of great importance to the water industry.