In this work, the high-pressure behavior of Pmn2(1)-Li2MnSiO4 and Pbn2(1)-Li2CoSiO4 is followed by in situ X-ray diffraction at room temperature. Bulk moduli are 81 and 95 GPa for Pmn2(1)-Li2MnSiO4 and Pbn2(1)-Li2CoSiO4, respectively. Regardless of the moderate values of the bulk moduli, there is no evidence of any phase transformation up to a pressure of 15 GPa. Pmn2(1)-Li2MnSiO4 shows an unusual expansion of the a lattice parameter upon compression. A density functional theory investigation yields lattice parameter variations and bulk moduli in good agreement with experiments. The calculated data indicate that expansion of the a lattice parameter is inherent to the crystal structure and independent of the nature of the transition-metal atom (M). The absence of pressure-driven phase transformation is likely associated with the incapability of the Li2MSiO4 composition to adopt denser structures while avoiding large electrostatic repulsions.