The miscibility properties of monolayers of two 1,2-diol derivatives, 1-palmitoylglycerol (1) and 1-hexadecylglycerol (2), have been studied at the air-water interface using grazing incidence X-ray diffraction (GIXD). While, at all pressures investigated, compound (I) exhibits only a NN-tilted rectangular structure, compound (2) has a phase transition to a NNN-tilted rectangular lattice at 18 mN/m. This transition pressure increases linearly with increasing proportion of compound (1). At the transition we observe a sudden increase in the long axis of the unit cell which is now the new tilt direction. Extrapolation of a plot of 1/cos(t) versus lateral pressure yields transition pressures to a phase with upright oriented molecules. Both pure compounds and all mixtures investigated show a non-zero lattice distortion at zero tilt angle indicating a probable herringbone packing of the molecules. A phase diagram is presented for the mixture of (1) and (2). (C) 1998 Elsevier Science B.V. All rights reserved.