Abstract: Apparatus for detecting minute quantities of moisture in a body of coolant has a support which confines a supply of particulate moisture sensitive material (such as CoBr.sub.2 or CoCl.sub.2) between a coolant-permeable filter and a light-transmitting seal. The coolant can contact the moisture sensitive material by way of the filter so that the material changes its color. A first diode serves to transmit light against the confined moisture sensitive material through the light-transmitting seal, and the incident light is reflected by the material to impinge upon a single generation diode which serves to transmit signals denoting the color of the moisture sensitive material to an evaluating circuit. The latter can activate an alarm system, interrupt the circulation of coolant and/or otherwise influence the coolant when the percentage of moisture (normally in parts per million) in the coolant reaches a predetermined maximum permissible value.