The mass-transfer rate in metallic triangular monolith channels with small protuberances on the channel walls was measured and compared with conventional, straight channels without bumps. The CO oxidation reaction was used to estimate the mass-transfer rates. Measurements were made at Reynolds numbers between 300 and 1400. The channels were 3.7 mm wide and 2.6 mm high and had a length of 96.5-152 mm. Two different bump heights were investigated: 1 and 1.2 mm. Moreover, the distance between the bumps as well as the distance from the inlet to the first bump was varied. It was found that the protuberances increase the mass-transfer rate, expressed as the Sherwood number. The increase was higher when the Reynolds number was increased. There was also a penalty of increased pressure drop. However, when the j(D) factor for mass transfer was compared with the friction factor f, it was found that the increase in mass transfer was generally higher than the increase in pressure drop.