t-Butyl peroxy-esters of cyclohexyl- and phenyl-phosphonic acids have been prepared and characterised. Di-t-butyl cyclohexyldiperoxyphosphonate can decompose by both heterolysis and homolysis of the peroxide bonds. The mono- and di-t-butyl peroxy-esters of phenylphosphonic acid decompose solely by heterolysis. In the case of di-t-butyl phenyldiperoxyphosphonate the peroxide bonds undergo heterolysis in two distinct stages. The relationship between the decomposition mechanism of t-butyl peroxy-esters and the pK_a values of the parent acids is discussed.