In this work, the external liquid-solid mass transfer properties of horizontal gas-liquid (G-L) and liquid-liquid (L-L) "slurry Taylor" flow are investigated. The catalytic particles (d(P) = 40-200 mu m) are transported in a cylindrical capillary (d(T) = 1.6-1.65 mm) in the form of a suspension in the aqueous liquid phase and kept in motion due to the recirculation patterns present in the liquid slugs. Ion exchange particles are used to follow the neutralization of dilute sodium hydroxide solutions in order to estimate the mass transfer coefficients. The influence of two phase velocity, particle size, slug length, and inert physical properties was investigated. No influence of slug length on the liquid-solid mass transfer coefficient was detected, and the particle size appears likewise to have no effect for particles larger than 50 mu m. The mass transfer coefficient depends mainly on overall flow velocity and an adequate circulation of solid particles in the liquid phase. For toluene-water L-L Taylor flow, particles agglomerate in the rear end of the droplet, and for the operating conditions (u(TP) = 1-6.5 cm s(-1)) applied in this study the Sherwood number was always found to be smaller than 2. For hexanol-water flow (u(TP) = 1-2.5 cm s(-1)) on the contrary, particles are well suspended in the lower part of the droplet, and higher Sherwood numbers could be found (2.5-8.75). The maximal possible flow rate to obtain stable Taylor flow conditions is, however, limited to quite low values in L-L Taylor flow. In G-L Taylor flow it is possible to work at higher two phase velocities (up to 28 cm s(-1)) under stable flow conditions and with homogeneously dispersed particles over the entire slug height. Sherwood numbers up to 15 were obtained under such conditions.