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Tho crystal and molecular structures of two phosphetane oxides were solved hy x-ray diffraction. 2,2,3-trimethyl-1-phenylphosphetane 1-oxide (C12POH17) crystallizes in space group P21/c with a = 10.582 Å, b = 12.680 Å, c = 10.299 Å, β = 119.03°. The phenyl group is planar, but the four-membered ring is puckered to some degree. The final R is 4.790 for 997 observed reflections. 2,2,3,4,4-pentamethyl-1-t-butylphosphetane 1-oxide (C12POH25) crystallizes in space group P21 with a = 6.133 Å, b = 12.174 Å, c = 9.047 Å, β = 96.42°. Since the refinement did not converge, only the gross structure is presented here. STATEMENT OF PERMISSION TO COPY In presenting this- thesis in, partial fulfillment of the require ments, for an advanced degree at Montana. State University, I agree that the Library shall make it freely available for inspection. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by my major professor, or, in his absence, by the Director of Libraries. It is understood that any copying or publication on this thesis for financial gain-shall not be allowed without my written permission