The technique of laser heating a thin metal foil was used to experimentally observe local phenomena in pool boiling CHF on a small flat surface. The experiments were performed with saturated water boiling on 25 mu m thick copper and titanium foils with heating surface diameters ranging from 2.27 to 9 mm. The number of simultaneously active nucleation sites depends on the heat flux and the size and material of the laser heated surface. A reduction in the heating surface diameter D-H moves the transition between one and several simultaneously active nucleation sites towards higher heat fluxes. CHF for small heating surfaces is lower than that for entirely heated surfaces. The variation of the ratio of bubble/vapour mass diameter to the heating surface diameter, D-B/D-H, With D-H is Similar to the variation of CHF with D-H. CHF is significantly influenced by the number of simultaneously active nucleation sites. The influence of heating of the surrounding surface on decreasing D-H, which was observed using two laser beams with different diameters was more intense at higher D-H On Cu foil than on Ti foil. (C) 2000 Elsevier Science Ltd. All rights reserved.