AbstractThe wetting ability of spray formulations is best assessed by a visual examination of the sprayed surfaces, but such a method is open to criticism in terms of observer bias and inaccuracy. By use of a panel of observers, the method has been examined to determine the magnitude of such errors and to find means of minimising them.Inexperienced observers showed considerable underestimation in determining the proportional areas of leaf surfaces wetted, particularly over the centre of the wetting range (30–80% of surface area wetted). As expected, the accuracy was improved by the use of charts and of a system of wetting classification that widened the assessment steps over the centre of the range.By including fluorescent tracers in the wetter solutions and photographing the wetted leaves under ultra‐violet light, the wetting was assessed also in terms of black areas on a white background. This proved to be much more accurate than direct leaf assessment and, although it cannot be recommended as a general method for wetting assessment, it is valuable for training observers by illustrating their errors.