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The thermal inactivation of a Citrobacter sp. ribonuclease (RNase) is subject to control by a number of factors. Low concentrations of naturally occurring polyamines such as spermidine and spermine, and certain analogs of these compounds, protect the enzyme from inactivation. Changes in ionic strength cause wide variations in the rate at which enzyme activity is lost. Additionally, depending on the type of ion added to the reaction mixture, the rate constant for enzyme inactivation-may either increase or decrease as the ionic strength is raised. Thermodynamic parameters were determined under a variety of experimental conditions for the thermal inactivation of this RNase. It was found in all of these cases that the entropy of activation is large and negative, implying that a gross change in enzyme conformation is not taking place. The concentration and identity of ions present and the amount of polyamine available to interact with this RNase determines the rate of loss, by thermal inactivation, of enzyme activity in this in vitro system. These factors therefore constitute a system whereby substrate hydrolysis may be controlled with time.