The behaviour of lawsonite has been studied by single-crystal diffraction to a maximum pressure of 9.82 GPa at room temperature. Lawsonite undergoes a phase transition at approximately 4 GPa from a structure with Cmcm symmetry to another orthorhombic phase with a P lattice. The full symmetry of this new phase could not be determined. The transition involves small spontaneous strains in each of the three lattice directions, but no significant volume strain. The transition character appears to be continuous, probably close to second-order. A further transition to a monoclinic phase occurs above 9.5 GPa. The EoS determined from all of the P-V data collected below 9.5 GPa for the two orthorhombic phases together has parameters V0=676.10(3) Å3, K0=122.1(4) GPa and K’=5.6(1). A fit of this new single-crystal data together with all the P-V-T data available provides the parameters of a thermal EoS of lawsonite: V0=676.03(6) Å3, KOT=122.8(8) GPa, K’=5.5(2), α0=2.63(7)x10-5 K-1, dK/dT=-0.018(5) GPa.K-1 for a reference temperature of 298 K.

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