The title compounds C₇H₈ClN₃O₂S, (I), and C₉H₁₀ClN₃O₄S_2, (II), both crystallize in monoclinic space group P2₁ /c with unit cell parameters (I) a = 7.9402(7), b = 10.6312(9), c = 11.7626(10), 脜, β = 99.271(5)掳, Z = 4 and (II) a = 5.1439(2), b = 9.0636(4), c = 27.1814 (7), 脜, β = 95.116(2)掳, Z = 4. In (I) the molecule consists of a 5-pyridine-4-yl group bonded to the carbon atom at the 5 position of (1, 3, 4) oxadiazole-2 thione hydrochloride monohydrate. The angle between the mean planes of the oxadiazole and pyridine rings is 9.6(6)掳. Crystal packing in (I) is stabilized by strong N–H路路路O hydrogen bonds in concert with a solvent water molecule and weak O–H路路路Cl, O–H路路路S, N–H路路路Cl intermolecular interactions. The crystal structure of compound (II) consists of 4 [5-ethylsulfanyl)-(1, 3, 4) thiadiazole-2-yl]-pyridinium perchlorate, (C₉H₁₀N₃S₂)⁺(ClO₄)⁻, cation–anion pairs, containing strong intermolecular N–H路路路O hydrogen bonds and weak C–H路路路O and N–H路路路O intermolecular interactions operating between the ionic species that form a cooperative hydrogen-bonded, infinite chain O–H路路路O–H路路路O–H network which generates a sheet motif structure in the unit cell. It is also supported by weak intermolecular Cg路路路Cg π–π and Cl–O路路路Cg π-ring interactions which gives additional support to molecular packing stability in the unit cell. Geometry optimized MOPAC AM1 computational calculations on each compound provides support to the structural features in their respective crystal structures.