Salts and cocrystals are multicomponent crystals that can be distinguished by the location of the proton between an acid and a base. At the salt end of the spectrum proton transfer is complete, and on the opposite end proton transfer is absent in cocrystals. However, for acid-base complexes with similar pK a values, the extent of proton transfer in the solid state is not predictable and a continuum exists between the two extremes. For these systems, both the ∆pK a value (pK a of base -pK a of acid) and the crystalline environment determine the extent of proton transfer. A total of 20 complexes containing theophylline and guest molecules with ∆pK a values less than 3 have been prepared, resulting in 13 cocrystals, five salts, and two complexes with mixed ionization states based on IR spectroscopy and single-crystal diffraction data. We propose modifications to the ∆pK a rule for selecting salt screen counterions that focus on the discovery of solid forms with useful physical properties rather than an arbitrary cutoff value for ∆pK a .