Sub-micrometric β-CoMoO 4 rods have been obtained after thermal treatment of CoMoO 4 ·H 2 O previously prepared by the precipitation method. The color of the sub-micrometric particles has been investigated through diffuse reflectance in the VIS-IR range and the spectra have been compared with those corresponding to samples with larger isotropic particles. The O 2− →Mo 6+ charge transfer band shifts to the UV region with decreasing the particle size, revealing a more covalent bond. This can be justified under the consideration of a higher proportion of surface-atoms in sub-micrometric particles that present lower coordination number. The piezochromic behavior of the sub-micrometric β-CoMoO 4 rods has been investigated. As the sub-micrometric size of the particles stabilizes the low-coordination phase (β-phase), the transition pressure to the α-phase is higher in comparison with that corresponding to larger particles. In addition, it was not possible to obtain a 100% transformation of the β-phase in the sample with sub-micrometric particles. A similar temperature influence on the transition pressure regardless of particle size has also been observed. Different optical properties and piezochromic behavior in β-CoMoO 4 sub-micrometric rods.