Solid-state NMR methods and ab initio calculations have been employed to investigate the structure of the trimethylphosphine (TMP)-Brønsted acid site complex in zeolite HY. 27Al/31P and 27Al/1H rotational echo double-resonance NMR experiments performed at -150°C were utilized to measure Al-P and Al-HB distances for the acid site-TMP complex of 3.95 (±0.05) and 2.8-3.1 Å, respectively, where HB is the Brønsted acid site proton. A more accurate measurement of the Al-HB distance was not possible since models that assume the presence of isolated Al-H spin pairs are not valid in this case. A P-HB distance of 1.40 (±0.02) Å was obtained by fitting the spinning sidebands in the 1H magic angle spinning (MAS) NMR spectrum. These internuclear distances are within the range of the Al-P, Al-HB, and P-HB distances obtained from ab initio calculations for the ion pair (IP) TMPH+-zeolite complex that is formed by transferring a Brønsted acid proton to TMP. In contrast to the 31P 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.