Mixed convection of a forced radially outward flow with secondary, buoyancy induced convection has been studied experimentally in an axisymmetric geometry, consisting of two differentially heated, horizontal, coaxial, circular plates with a diameter of 50 cm and a mutual distance of 2 cm. Through the centre of the upper plate, a laminar forced airflow is supplied. Particle image velocimetry, flow visualisation and local temperature measurements have been used to study the onset of thermal instability as a function of the inflow and temperature difference, and to validate the numerical results obtained in Part I of this paper. (C) 2000 Elsevier Science Ltd. All rights reserved.