Liquid-to-particle mass transfer effects in a string pellet minireactor were studied by applying the diffusion-controlled copper dissolution technique. Structured beds were formed with cylindrical particles in spiral and vertical configurations and operated with liquid and gas-liquid feeds in upflow mode. Single-and two-phase flow experiments were conducted to study the effects of the gas and the liquid superficial velocities on the liquid-to-particle mass transfer coefficient. Computational fluid dynamics calculations were performed to study the effect of the arrangement of the particles in the reactor tube on the liquid-to-particle effective mass transfer coefficient for the single-phase flow. Correlations of the mass transfer coefficient with the operating conditions were derived.