Abstract: This patent discloses the use of poly-substituted isothiouronium salts (T1 where R1, R2, R3, R4=hydrogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, aryl, heteroaryl, etc. and X−=Cl−, Br−, NO3−, CH3CO2−, or any other common anion), poly-substituted guanidinium salts (G1 where R1, R2, R3, R5, R6=hydrogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, aryl, heteroaryl, etc. and X−=Cl−, Br, NO3−, CH3CO2−, or any other common anion), or mixtures of two or more of the above compounds, as the biocidal component of microbiocidal or anti-fouling formulations.