Different　crystallographic　forms　of　MnO2　nanorods　(alpha,　beta,　gamma,　and　mixed　phase)　were　prepared　using　a　hydrothermal　method　with　different　reaction　times　or　reagents.　The　samples　obtained　were　characterized　by　X-ray　diffractometry　and　transmission　electron　microscopy.　The　catalytic　activities　of　the　different　phases　of　MnO2　nanorods　during　thermal　decomposition　of　ammonium　perchlorate　(AP)　were　investigated　using　thermogravimetric　analysis.　The　catalytic　activity　of　MnO2　was　mainly　size-related　and　not　phase-controlled.　Different　phases　of　MnO2　with　similar　average　diameters　showed　similar　catalytic　activities　and　differed　from　the　other　catalytic　activities　of　different　crystallographic　forms　of　MnO2.　Mixed-phase　MnO2,　which　presented　the　smallest　average　diameter　of　9.5　nm,　showed　the　best　catalytic　activity　and　lowered　the　AP　peak　temperature　by　132.4　degrees　C.　(C)　2015　Elsevier　Ltd　and　Techna　Group　S.r.l.　All　rights　reserved.