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Structure-activity relationships are important for the design of biocides and sanitizers. During the spread of resistant strains of pathogenic microbes, insights into the correlation between structure and activity become especially significant. The most commonly used biocides are nitrogen-containing compounds; the phosphorus-containing ones have been studied to a lesser extent. In the present study, a broad range of sterically hindered quaternary phosphonium salts (QPSs) based on tri- tert -butylphosphine was tested for their activity against Gram-positive ( Staphylococcus aureus, Bacillus cereus , Enterococcus faecalis ) and Gram-negative ( Escherichia coli , Pseudomonas aeruginosa ) bacteria and fungi ( Candida albicans , Trichophyton mentagrophytes var. gypseum). The cation structure was confirmed to determine their biological activity. A number of QPSs not only exhibit high activity against both Gram-positive and -negative bacteria but also possess antifungal properties. Additionally, the hemolytic and cytotoxic properties of QPSs were determined using blood and a normal liver cell line, respectively. The results show that tri- tert -butyl( n -dodecyl)phosphonium and tri- tert -butyl( n -tridecyl)phosphonium bromides exhibit both low cytotoxicity against normal human cells and high antimicrobial activity against bacteria, including methicillin-resistant strains S. aureus ( MRSA ). The mechanism of QPS action on microbes is discussed. Due to their high selectivity for pathogens, sterically hindered QPSs could serve as effective tunable biocides.