The thermal decomposition of commercial chlorates was studied t o obtain maximum yields and efficiency and develop a process for manufacturing perchlorates. NaClO sub 3 thermally decomposed to a mixed salt product of 62% NaCl sub 4, 13% NaClO sub 3, and 25% NaCl with an efficiency of about 80%. Highest yields and efficiencies occurred at relatively low temperatures and long times. Trace impurities may adversely catalyze the reactions. A critical temperature existed above which a batch would spontaneously decompose completely to O and NaCl; the critical temperature was a function primarily of the mass/area ratio and the presence of catalysts or catalytic surfaces. Commercial and purified NaClO sub 3 gave substantially the same results. KClO sub 3 was converted to a mixed salt containing 68% KClO sub4, 9% KClO sub 3, and 23% KCl with an optimum efficiency of 83%. The conversion process was more difficult to control than that with NaClO sub 3. NaClO sub 3-KCl mixtures produced perchlorate at higher conversion efficiencies than KClO sub 3; the highest efficiency was 88.4%. Fused quartz and Vycor were almost inert catalytically when in contact with chlorates under conversion conditions. Metalic surfaces promoted adverse catalysis. Some alloys were passivated to behave like quartz. NaCl in the conversion product was separated by leaching the H sub 2 O. NaClO sub 3 and NaClO sub 4 could not be separated by any simple method. NH sub 4 ClO sub 4 was prepared by metathesis from NH sub 4 Cl and the mixed-salt product of the thermal conversion of NaClO sub 3. KClO sub 4 may be obtained by metathesis from KCl in an analogous manner.