This paper describes the development of a novel proximity gauging technique for soft deposits, exploiting a siphon effect. This non-contact technique has the capability to measure the thickness of soft deposits on a surface in situ and in real time. A theoretical model of performance has been developed and its validity demonstrated by calibration experiments. Local thickness measurements of soft deposits such as whey protein deposit, supermarket butter and sticky foam on metal surfaces had an accuracy of +/-20 mum for a deposit thickness of 500 to 1000 mum. The potential of this technique for on-line monitoring of fouling and cleaning processes in liquid environments is demonstrated by preliminary studies of alkali (NaOH) cleaning of whey protein deposits from stainless steel surfaces.