We report on the short-time dynamics in colloidal mixtures made up of monomers and dimers highly confined between two glass-plates. At low concentrations, the experimental measurements of colloidal motion agree well with the solution of the Navier-Stokes equation at low Reynolds numbers, which takes into account the increase of the drag force on each particle due to wallparticle hydrodynamic forces. We find that the ratio of the short-time diffusion coefficients of the monomer and that of the center of mass of the dimer remains independent of both the total packing fraction and the dimer molar fraction up to concentrations near to the crystallization transition. The same physical scenario is observed for the ratio between the parallel and perpendicular components of the short-time diffusion coefficients of the dimer. This dynamical behavior is corroborated by means of Molecular Dynamics computer simulations that explicitly include the particle-particle hydrodynamic forces induced by the solvent. Thus, our results point out toward that the effects of the particle-particle hydrodynamic interactions on the diffusion coefficients are identical and, thus, factorable in both species.Introduction. Many phenomena observed in colloidal dispersions resemble those of atomic systems. However, the colloidal dynamics exhibits special features due to the solvent-mediated forces typically known as hydrodynamic interactions (HI) [1]. Contrary to direct particleparticle interactions, HI can be tuned, but never completely screened or switched off. In a simple physical picture, HI can be understood as follows. The motion of a given colloidal particle induces a flow field in the solvent which is felt by the surrounding colloids. Thus, the motion of one colloidal particle causes a solvent-mediated force on the neighboring colloidal particles.In contrast to the static counterpart, the colloidal dynamics is far from being completely understood. The reason is partially related with the fact that the colloidal dynamics extends over a wide range of temporal and length scales due to the enormous difference in size and mass between the colloids and the solvent molecules, giving rise to complex and long-ranged HI [1,2]. The latter ones lead to non-trivial coupling among colloids that extends over many mean-interparticle distances [1,2]. The understanding of HI is thus of relevance not only in physics, but also in several branches of science, such as biology, since phenomena like hydrodynamic synchronization in either biological systems (sperm, cilia, flagella) [3,4] or active fluids [5], and the dynamics of microswimmers [6,7] can only be explained in terms of hydrodynamic forces.During the last few decades, the study of the hydrodynamic coupling between two or three spherical colloids [8,9] or a spherical colloid near a wall [10][11][12] has been