Sodium and potassium tetraphenylborates were examined by solid-state Na-23 and K-39 NMR spectroscopy. Analyses of solid-state NMR spectra obtained at 4.70, 11.75, and 19.60 T yielded the following Na-23 and K-39 NMR parameters: Na[BPh4], \C-Q\ = 1.24 +/- 0.05 MHz, eta(Q) = 0.0 +/- 0.1, delta(iso) = -45.6 +/- 0.5 ppm, and Omega = 14 +/- 2 ppm; K[BPh4], \C-Q\ = 1.32 +/- 0.05 MHz, eta(Q) = 0.0 +/- 0.1, and delta(iso) = -92 +/- 1 ppm. In both Na[BPh4] and K[BPh4], the electric field gradient and chemical shift tensors at the metal site are axially symmetric, in agreement with the crystallographic symmetry. Extensive quantum mechanical calculations were performed for Na[BPh4] and K[BPh4] as well as for a large number of model cation-pi systems containing Na+ and K+ ions and common aromatic compounds. Experimental and theoretical studies confirm that a highly shielded environment at the metal cation site is a characteristic feature for cation-pi interactions, making it useful as a NMR signature for identifying cation-pi interactions in proteins and nucleic acids. In this study, powder X-ray diffraction spectra for Na[BPh4] and K[BPh4] were also reported.