A tetraalkylhydrophosphorane having three methylene groups instead of three equatorial oxygen atoms of a 1-hydro-5-carbaphosphatrane was synthesized by the reaction of the corresponding phosphonium salt with LiAlH4. A tetraalkylchloro- phosphorane was obtained from the phosphonium salt. Their X-ray crystallographic analyses showed that they have nearly ideal trigonal bipyramidal structures. Their bond lengths and angles as well as apical coupling constants showed that they have usual apicophilic arrangement. The tetraalkylhydrophosphorane gave two phosphine oxides, one of which was formed via homolytic bond cleavage of P-methine carbon bonds under daylight. Another phosphine oxide was estimated to be its isomer formed via the P-benzyl bond cleavage. The reaction of the chlorophosphorane with Me3Al afforded the corresponding pentaalkylphosphorane with the methyl group.