Variable-temperature single-crystal X-ray diffraction using a synchrotron X-ray source has allowed the mechanism of negative thermal expansion in the pure silica zeolite IFR to be studied in greater detail than was previously possible over the temperature range 30-557 K. The results have allowed the changes in average atomic position with temperature to be measured and the structural features that are important in negative thermal expansion to be identified. The structure of zeolite IFR can be split into two regions: columns of fused rings that expand with temperature and the intercolumn regions, which tend to contract on heating. These competing changes combine to produce a material that contracts parallel to the crystallographic a and b axes and expands in the c-direction. Correlations between zeolite structure and thermal expansivity are also reported.