Angiosperm mitochondrial (mt) genes are generally slow-evolving, but multiple lineages have undergone dra-matic accelerations in rates of nucleotide substitution and extreme changes in mt genome structure. Whilemolecular evolution in these lineages has been investigated, very little is known about their mt function. Somestudies have suggested altered respiration in individual taxa, although there are several reasons why mt var-iation might be neutral in others. Here, we develop a new protocol to characterize respiration in isolated plantmitochondria and apply it to species ofSilenewith mt genomes that are rapidly evolving, highly fragmented, andexceptionally large (~11 Mbp). This protocol, complemented with traditional measures of plantfitness, cyto-chromecoxidase activity assays, andfluorescence microscopy, was also used to characterize inter- and in-traspecific variation in mt function. Contributions of the individual“classic”OXPHOS complexes, the alternativeoxidase, and external NADH dehydrogenases to overall mt respiratoryflux were found to be similar to previouslystudied angiosperms with more typical mt genomes. Some differences in mt function could be explained by inter-and intraspecific variation. This study suggests thatSilenespecies with peculiar mt genomes still show relativelynormal mt respiration. This may be due to strong purifying selection on mt variants, coevolutionary responses inthe nucleus, or a combination of both. Future experiments should explore such questions using a comparativeframework and investigating other lineages with unusual mitogenomes