The genus Thiobacillus was previously considered to be a member in this family but was reclassified into the order Nitrosomonadales at the same time that the Hydrogenophilales were removed from the Betaproteobacteria to form the class Hydrogenophilalia.

[3] The presence of DNA from (and potentially live cells of) thermophilic bacteria in the ice suggests that a geothermal system could exist beneath the cold water body of Lake Vostok, or simply that non-thermophilic strains of Hydrogenophilus exist and were present in the ice.

The class Hydrogenophilalia in the Bacteria was circumscribed in 2017 when it was demonstrated that the order Hydrogenophilales was distinct from the Betaproteobacteria on the basis of physiology, biochemistry, fatty acid profiles, and phylogenetic analyses on the basis of the 16S rRNA gene and 53 ribosomal protein sequences concatenated using the rMLST platform for multilocus sequence typing.

The very high proportion of ω-cyclohexyl fatty acids (specifically C19:0 cyclo and C17:0 cyclo) versus straight counterparts was a major distinguishing feature versus the Betaproteobacteria, and is probably involved in ensuring membrane stability at high growth temperatures.

The autotrophic members of the class do not use carboxysomes to concentrate carbon dioxide or improve RuBisCO efficiency as a carboxylase versus an oxygenase.