In the title salt, C(13)H(19)N(2) (+)·BF(4) (-), an ionic liquid, the butyl and ethyl substituents bonded to the N atoms of the imidazole ring [r.m.s. deviation = 0.019 (1) Å] adopt equatorial positions. The crystal structure exhibits slipped π-π inter-actions between the imidazole and benzene rings of neighbouring mol-ecules [centroid-centroid distance = 3.529 (2) Å]. In the tetra-fluoro-borate anion, the B and F atoms are disordered over two sets of sites with site-occupancy factors of 0.813 (7) and 0.187 (7).