The title compounds, 2-(4-bromophenyl)-1,2-dihydropyrimido[1,2-a]benzimidazol-4-(3H)-one, C(16)H(12)BrN(3)O, (IVa), and 4-(4-methylphenyl)-3,4-dihydropyrimido[1,2-a]benzimidazol-2-(1H)-one, C(17)H(15)N(3)O, (Vb), both form R(2)(2)(8) centrosymmetric dimers via N-H...N hydrogen bonds. The N...N distance is 2.943(3)A for (IVa) and 2.8481(16)A for (Vb), with the corresponding N-H...N angles being 129 and 167 degree, respectively. However, in other respects, the supramolecular structures of the two compounds differ. Both compounds contain different C-H...pi interactions, in which the C-H...pi(centroid) distances are 2.59 and 2.47A for (IVa) and (Vb), respectively (the latter being a short distance), with C-H...pi(centroid) angles of 158 and 159 degree, respectively. The supramolecular structures also differ, with a short Br...O distance of 3.117(2)A in bromo derivative (IVa), and a C-H...O interaction with a C...O distance of 3.2561(19)A and a C-H...O angle of 127 degree in tolyl system (Vb). The dihydropyrimido part of (Vb) is disordered, with a ratio of the major and minor components of 0.9:0.1. The disorder consists of two non-interchangeable envelope conformers, each with an equatorial tolyl group and an axial methine H atom.