The photochromic tracer technique was used to study the turbulent boundary layer over two transverse V-shaped grooves. The mean velocity profiles above the individual grooves were parallel to those seen in the outer regions for smooth surfaces, but displaced downwards. Also, the turbulence intensity just above the grooves was higher than the corresponding smooth surface values. These results suggest that enhancement of heal and mass transfer rates by transverse grooves is due to energetic bursts which effectively replace the liquid within the groove, with a contribution from an apparent increase in burst frequency.