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The Western White along with the other two Pontia whites, form a group of very similar species. The Spring White (Pontia sisymbrii) is easiest to distinguish; the dark vein markings on the hindwing underside are not connected laterally, and it occurs only in montane woodlands and extreme northern Alberta. The Western and Checkered White (P. protodice) are more difficult to separate. Males of the Western are more heavily marked than the Checkered, particularly on the underside. Females of both species have heavier markings than the males, but these markings are brown in P. protodice, not charcoal or black. Another characteristic is found on the underside of the forewing apex: P. occidentalis has the dark submarginal band connected with dark markings along the veins to the wing margin, P. protodice has only pale yellow scales here. Individuals of the spring generation are smaller and have darker underside markings. Alberta populations are the nominate subspecies.

The eggs are a dull orange colour. Mature larvae are blue-grey with a yellow dorsal and lateral stripe, and many black dots. Pupae are light grey with small black spots, and overwinter (Layberry et al. 1998, Guppy & Shepard 2001). Males display hilltopping behaviour, and can sometimes be found on mountain peaks over 3000m. In the mountains, there is only one brood that emerges in late spring, but there are two (sometimes a partial third) broods at lower elevations (Bird et al. 1995). This is our most common species of Pontia.

Although this is often a very common species, there are no larval foodplant records for Alberta. Wild mustards (Brassicaeae) are recorded elsewhere, including Lepidium, Cleome, Draba, Sisymbrium and Thlaspi (Layberry et al. 1998, Guppy & Shepard 2001).