An automatic Langmuir-type surface balance for studies in a UV or ionising radiation field has been designed. The apparatus is suitable for measurements of the following functions: pressure p=p(A), potential V=V(A), area/molecule A=A(t)p, V=V(t)p, p=p(t)A, V=V(t)A where t denotes time, in the temperature range 278-313K maintained to within +or-0.5K. The sensitivity of the balance is 0.1 mu N cm-1 and this value is limited chiefly by the quality of the anti-vibration arrangement. A new design of the surface electrode minimises the influence of radiation from the ionising source upon the film. Examples of monolayer behaviour and other tests are presented.