Hydrolysis of either Bi(HSal4Me)3 (A) or [Bi22O26(HSal4Me)14] (B) in DMSO gave the 4-methylsalicylate-substituted bismuth oxido cluster [Bi38O45(HSal4Me)24(DMSO)14(H2O)2]∙4H2O (1∙4H2O) which crystallizes in the triclinic space group P1¯ with cell parameters of a = 20.7214(5) Å, b = 20.9654(6) Å, c = 22.2128(6) Å, α = 100.867(2)°, β = 114.108(2)°, γ = 107.895(2)° and V = 7815.1(4) ų. Studies on the hydrolysis of A were performed using 1H DOSY NMR spectroscopy and electrospray ionisation Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry which both show the formation of several larger species under ambient conditions in the presence of moisture. Furthermore, ESI- FTICR MS of cluster B shows the formation of mainly {Bi23O26} species which indicates partial dissociation of cluster B in solution. Additionally, the three different bismuth species containing one (A), 22 (B) or 38 (1∙4H2O) bismuth atoms per molecular formula were tested for their microbiological activity against three strains of Helicobacter pylori (251, 26695 and B128). A minimum inhibitory concentration (MIC) value of 6.25 µg/mL for the mononuclear bismuth complex A was obtained whereas the bismuth oxido cluster B shows a lower activity with MIC values between 25 μg/mL and 50.0 μg/mL. The activity of 1∙4H2O is comparable to commercial remedies based on antimicrobial bismuth subsalicylate (BSS; 12.5 ug/mol).