In Arabidopsis thaliana the putative mitochondrial RNA helicases PMH1 and PMH2 are members of the large DEAD-box protein family. Our previous characterization of these proteins revealed that PMH1 and/or PMH2 are part of high molecular weight complexes. Now T-DNA insertion lines were established and characterized for each of these genes. Immunodetection analysis of cell suspension cultures established from pmh1-1 and pmh2-1 mutants revealed that indeed both DEAD-box proteins are detectable in large protein complexes with PMH2 being much more abundant than PMH1. In plants the knockout of PMH2 leads to reduced group II intron splicing efficiency. In addition the steady-state levels of several mature mitochondrial mRNAs are decreased while transcription is not influenced. This molecular phenotype suggests that PMH2 acts at the posttranscriptional level with a potential function as RNA chaperone required for formation or maintenance of complex RNA secondary structures of introns rather than a direct role in splicing. In contrast, the investigation of a pmh1-1 knockout line did not reveal any influence of this protein on processing and abundance of mitochondrial transcripts.