Parmotrema masonii is characterized by the short cilia (up to 0.7 mm), the presence of salazinic and hypoconstictic acid and the absence of propagules.
Related species include P. delicatulum (longer cilia and with different chemistry), P. flavescens (isidiate), P. mirandum, P. nylanderi and P. pectinatum (sorediate).
The holotype of P. masonii has the upper surface strongly rugose (resembling that of P. madylinae), short and tapered cilia. The isotype agrees very well with the holotype, except that it has slightly shorter ascospores 13.5–15 µm long and apothecia frequently irregularly “coronate” with black dots, probably formed by some parasitic fungus. The paratype received from CTES presents several differences in the relation to the holotype and isotype: 1) larger, lacerate apothecia (up to 8 mm in diameter); 2) smaller ascospores, 10–12.5 × 6–7.5 µm, episporium 0.5–1.0 µm; 3) upper surface only slightly rugose and 4) cilia usually thinner and longer (up to 1.5 mm). Nevertheless, the conidia and overall morphology are similar.