Antimicrobial photodynamic therapy (APDT) is a novel treatment method for the destruction of microorganisms. APDT uses the combined effect of a photosensitising drug, light and molecular oxygen to bring about a toxic effect. The work presented in this study investigates the photodynamic antibacterial efficacy of a series of symmetrical cationic phenothiazinium salts. The photosensitisers investigated were lipophilic derivatives of methylene blue, where the methyl groups of methylene blue are replaced with longer alkyl chains from ethyl to n-hexyl.