Solid-state NMR methods and ab initio calculations have been employed to investigate the structure of the trimethylphosphine (TMP)-Bronsted acid site complex in zeolite HY. Al-27/P-31 and Al-27/H-1 rotational echo double-resonance NMR experiments performed at -150 degrees C were utilized to measure Al-P and Al-H-B distances for the acid site-TMP complex of 3.95 (+/-0.05) and 2.8-3.1 Angstrom, respectively, where H-B is the Bronsted acid site proton. A more accurate measurement of the Al-H-B distance was not possible since models that assume the presence of isolated Al-H spin pairs are not valid in this case. A P-H-B distance of 1.40 (+/-0.02) Angstrom was obtained by fitting the spinning sidebands in the H-1 magic angle spinning (MAS) NMR spectrum. These internuclear distances are within the range of the Al-P, Al-H-B, and P-H-B distances obtained from ab initio calculations for the ion pair (IP) TMPH+-zeolite complex that is formed by transferring a Bronsted acid proton to TMP. In contrast to the P-31 MAS NMR spectra, which indicated that the only stable species was TMPH+, ab initio calculations on small cluster models predicted that the neutral complex should be more stable than the IP complex. However, use of a larger zeolite fragment in the calculations enhanced the relative stability of the IP structure.