This paper describes the true geometry of the pentacoordinated SiO4F- units found inseveral all-silica zeolite systems, as determined by ab initio density functional calculations.The SiO4F- units in the two zeolites (sodalite and ferrierite) studied contain Si−F bonds of1.71 and 1.76 Å in length, and all the Si−O bonds in the units are elongated with respectto the Si−O bonds in the tetrahedral SiO4 unit typically found in a zeolite. All the bondangles in the SiO4F- unit agree with those expected for a regular trigonal bipyramidal XY5unit. The calculated structure is shown to reproduce accurately an averaged structural modeldetermined by standard crystallographic methods. The approach we have developed in thiswork has general implications for the solution of complex local structure problems in solids.