The preparation of quaternary ammonium and arsonium pentafluorosilicates is described. It is shown that SiF5 is a relatively weak acceptor, forming an unstable 1:1 ammonia adduct and interacting only weakly with other Lewis bases. The infrared and Raman absorptions due to SiF5 are not inconsistent with a trigonal-bipyramidal geometry. Many spectroscopic similarities with phosphorus pentafluoride are apparent. © 1969, American Chemical Society. All rights reserved.