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Diffuse scattering in titanite has been measured at three temperatures, 0.951Tc, 1.053Tc and 1.177Tc, using synchrotron radiation. Tc = 487 K is the temperature of the antiferroelectric paraelectric phase transition. Charge-coupled device (CCD) intensity measurements were centred about the 01 reciprocal-lattice position (space-group setting P21/a) and extended beyond the neighbouring 11 and 1 fundamental positions. Planar diffuse scattering intensity is observed normal to [100] with a lens-shaped maximum centred at 01. On approaching Tc from above, the intensity of the maximum at 01 increases, while intensity scattered into the planes decreases at large distances to the critical zone-boundary position. The intensity distribution is interpreted in terms of a two-dimensional spin-1/2 Ising model, in which individual spin states represent the collective displacement of octahedrally coordinated Ti atoms parallel to [100].