Enzyme immobilization in porous silica particles is used to improve enzyme function in biocatalytic applications. Here, we study the effective protein concentration and rotational mobility of lipase and bovine serum albumin in the pores, when confined in five types of mesoporous silica particles with different pore and particle sizes, exploiting the intrinsic UV-vis absorption and fluorescence anisotropy of the tryptophan residues. For all investigated combinations of proteins and particles, the steady-state anisotropy is higher than for the same protein in free solution, indicating a slower protein rotation inside the pores. The retardation is stronger in more narrow pores, but the proteins can still move, and there is no dependence on the particle size. The average number of proteins per particle, Nprot, varies with the particle diameter, D, as Nprot ∼ D2.95±0.02 for both proteins, which is close to the scaling D3.0±0.1 for the available pore volume. This observation indicates that both proteins are distributed evenly throughout the particles and rules out that the proteins are only externally bound to the particle surface. Secondly, the concentration of the protein in the pores depends on the pore and protein size but not on the particle size and corresponds to volume fractions in the range of 20-60%.