The scalar field (heat or species concentration) of a small‐source laminar starting plume was measured at a single dimensionless buoyancy injection rate for two values of the ratio of momentum diffusivity to scalar diffusivity, Pr=7 and 300. Digital image processing was used to extract concentration contours from the interferograms, which considerably reduced the data reduction time and permitted an improved resolution. The effect of Pr was clearly visible in the scalar contours, especially in terms of the entrained fluid that is found to be undiluted far inside the cap of the higher Pr. The scalar contours were not self‐similar when a single length scale was used, but the data collapsed well using a two‐length scale normalization. The convective velocity of the cap was also measured for the two cases, suggesting a possible logarithmic effect of Pr. An analysis confirms the speculation. It was also found that about 37% of the total injectionrateinto the starting plume is convected into the cap. This work is the first experimental investigation demonstrating the effect of change in Pr on the motion of small buoyancy‐source laminar gravitational convection phenomena.