A method is proposed for installing monoliths (2) each formed of a ceramic block having a multiplicity of mutually parallel channels wherethrough the reaction gas mixture of a heterogeneously catalyzed gas phase reaction can flow in a reactor (1) for conducting heterogeneously catalyzed gas phase reactions, wherein said monoliths (2) are stacked side by side and on top of each other in the reactor interior, wherein the monoliths are sealed off from each other and from the inner wall of said reactor (1) by mats (3) each comprising an intumescent mat which before installation in said reactor (1) were completely enveloped in a polymeric film, wherein the interior enclosed by the polymeric film and containing said mat (3) is evacuated and wherein the interior enclosed by the polymeric film and containing said mat (3) is devacuated after installation in said reactor (1).