An instrument for monitoring the characteristics of a liquid comprises a source of radiation (6) which is transmissible through the liquid and a detector (2) for detecting radiation emerging from the liquid. The detector (2) is responsive to components of the emerging radiation which are affected by variations in the characteristics of the liquid. The source (6) and detector (2) are arranged such that the radiation is transmitted through at least one free surface of the liquid (5) which is supported at least in part by surface tension of the liquid.