Methods and systems for temperature-controlled, on-site generation of peracids, namely peroxycarboxylic acids and peroxycarboxylic acid forming compositions are disclosed. In particular, methods for using an adjustable biocide formulator or generator system overcome the limitations of temperature on the kinetics of the peracid generation and/or peracid decomposition inside an adjustable biocide formulator or generator system. The methods include the controlling of the temperature of at least one raw starting material, namely water, to improve upon methods of on-site generation of peracids. The methods allow for the generation of user-selected chemistry without regard to the ambient temperatures of the raw starting materials and/or the biocide formulator or generator system.