Free convective mass transfer in open upward-facing cylindrical cavities was studied experimentally using the limiting diffusion current technique. The mass transfer rate for the total vertical cavity with all surfaces active is higher (except for the shallowest cavity) than for cavities with walls only active; this is due to the cavity base flow disturbing the more fully developed boundary layers on the longer vertical surfaces. The present correlation for vertical cavities based on characteristic length L-w (surface area/perimeter projected onto a horizontal plane) Sh(Lw) = 0.559Ra(Lw)(0.265) for Ra-Lw in the range from 2 x 10(7) to 1.2 x 10(10) and Sc is an element of (2050, 2300) was found to be applicable for cavities with various height/diameter ratios (0.22-2.8). Re-evaluation of previous experimental data of Somerscales and Kassemi also gave excellent agreement with this equation. The flow development after sudden switch on of the limiting current potential was observed. The how emerging from the mouth of vertical cavities with either active or inactive base was similar, showing a visible pulsing which was also noticed on the current transients. The current transients for deeper vertical cavities showed the slow build up of the steady state diffusion layer, whereas for shallow cavities an undershoot and overshoot, typical of up-facing horizontal surfaces, was observed.