Particular nitrogen (in lithium), oxygen (in Li//1//7Pb//8//3), hydrogen and carbon activities need to be kept below critical levels for satisfactory operation with the structural materials chosen for fusion reactor cooling circuits. To reduce the metalloid content in lithium and lithium-lead below the critical values the development of on-line monitoring systems is essential. The hydrogen concentration in liquid lithium has been determined by measurement of the equilibrium pressure using alpha-iron as a sensor material. The measurements done in static and dynamic lithium show that a good relationship exist between the measured partial pressure and the hydrogen content of the lithium. The carbon concentration in the circulated lithium is determined by the use of a Harwell type carbon meter (HCM). The oxygen and nitrogen concentration in the static lithium-lead is measured with electrochemical sensors. Preliminary results of these measurements are reported.