The Schlieren effect is a consequence of light inflexions altering the spectrophotometric measurements that are due to the formation of optical artefacts such as mirror and lenses inside the flowing sample. It may influence signal-to-noise ratio, sample throughput and measurement repeatability in spectrophotometric flow-based procedures. It was critically examined, in order to provide guidelines for system design. Results demonstrate that the magnitude of the Schlieren effect is governed by the combined influence of two components. The first is related to loss of light by reflection at sharp liquid interfaces and prevails under poor mixing conditions. In this situation the transmitted light becomes more intense and somewhat erratic. The second is related to light refraction in regions with pronounced refractive index gradients, and prevails under good mixing conditions. It can lead to positive or negative modifications of the transmitted light, the direction being dependent on the relevance of these components. The importance of the solubility of the chemical species involved, the possibility of interactions with the inner wall of the tubing, and the feasibility of dual-wavelength spectrophotometry for compensating purposes are discussed.