This paper presents the results of an experimental investigation on longitudinal mass diffusion in the tubes roughened by equidistantly-spaced grooves during oscillatory flows of Reynolds number 210-13,000 and Womersley number = 3.76-10.64. The experiments were conducted in tubes of 6 mm nominal ID using a soluble contaminant of molecular diffusion coefficient = 2.78 x 10(-5) cm(2)/s as the tracer. The experiments show that, in comparison with the smooth one, the grooved tubes could yield an augmentation of up to about two orders of magnitude in effective diffusion rate for our investigation. The present results indicate that, for high Reynolds number flows, the ratio of the volume of core region to that of grooves influence the longitudinal diffusion efficiency in a tuning behavior, the grooved tube in which the core region is equal to grooves in volume performs best in enhancing the axial mass diffusion. (C) 2001 Elsevier Science Ltd. All rights reserved.