The present study aims at understanding the molecular structure and dynamics of the acidic polymer poly(vinylphosphonic acid) (PVPA) blended with the basic polymer poly(2,5-benzimidazole) (ABPBI) under anhydrous conditions. The extent of the acid - base complexation is probed at different mixing ratios. Advanced 1H, 13C, and 31P solid-state NMR methods are used to investigate the structural features in these systems. In addition, molecular dynamics is studied by variable-temperature 1H magic angle spinning and one-dimensional double-quantum NMR methods. Many different types of hydrogen-bonding are identified in the acid - base complexes. Addition of the acidic PVPA to the basic ABPBI changes the molecular packing arrangements of the ABPBI moieties with hydrogen-bond formation as the driving force. The complex with a 1:1 mixing ratio has the lowest activation energy for proton mobility, and at the same time contains the most structured hydrogen-bonded protons. The results show that molecular-level mixing is achieved for the complexes.