The unit-cell parameters of MoO3
(space group Pbnm
) have been studied as functions of pressure from 0 to 7.41 GPa in a single-crystal experiment involving measurements at 28 different pressure values. A diamond-anvil high-pressure cell mounted on a four-circle X-ray diffractometer was used. The compressibility of this layer structure was found to be very anisotropic. The mean linear compressibilities in the three main directions of the unit cell have the following values: a
(0-7.41 GPa) = - 3.2 × 10-3
(0-0.49 GPa) = - 3.5 × 10-2
(1.54-7.41 GPa) = - 1.1 × 10-2
, and c
(0-7.41 GPa) = - 3.35 × 10-3
is the length of a lattice vector d
at the hydrostatic pressure Pi
. The compressibility in the  direction, i.e.
normal to the layers, has a non-linear region and is by far the largest one. On the other hand, the compressibility in the  direction, which is the direction of the chains of oxygen coordination polyhedra within each layer, is very small.