Racomitrium is usually recognized by its strongly sinuose leaf cells. There are some Grimmia species with sinuose lower leaf cells and these may be difficult to distinguish from depauperate collections of Racomitrium without examining stem cross‑sections: Racomitrium lacks a central strand, nearly all Grimmia species have one. The leaf apices in Central America collections of Racomitrium are especially variable. In both species some collections are fully muticous, others always have long, hyaline tipped leaves, while still others have both types of leaves.

The Racomitrium peristome has a basal membrane and variably 2–3-cleft teeth. In some species the teeth are filiform and rarely united by trabeculae. In others they are weakly cleft and have well-developed trabeculae that hold the divisions together. The peristomes with free, filiform divisions approach those of Ptychomitrium, while the weakly cleft teeth are similar to strongly cribrose Grimmia peristomes. The peristomial basal membrane in Racomitrium is often obscured by a prostome that occasionally exhibits a cell pattern indicative of an OPL or exostomial origin. Although peristome and calyptra form are important in assessing generic relationships within the Grimmiaceae, characters from these structures do not correlate in Racomitrium: peristomial evidence places it between Coscinodon and Ptychomitrium, while calyptral evidence places it between Coscinodon and Grimmia.

Racomitrium languginosum (Hedw.) Brid. was recently collected in Costa Rica (San José: Holz & Schäfer-Verwimp CR99-1212, GOET, MO). It is distinguished from other Central American Racomitrium species by its leaves that have long, erose-dentate, conspicuously decurrent hairpoints.