[2] Byssoloma xanthonicum has a thin, non-corticate (without a cortex), and continuous thallus (the main body of the lichen) that is cream-coloured with a dull appearance.

These apothecia are round but often lobate (lobed) in shape, with a flat, dull, dark brown disc that is not pruinose (powdery).

The hymenium (spore-bearing layer) is 25–35 μm high, hyaline (translucent) but becomes brownish at the base due to diluted pigment from the hypothecium.

[2] The thallus of Byssoloma xanthonicum fluoresces a yellow-orange colour when lit with a ultraviolet light, owing to the presence of the compound lichexanthone.

Compared to B. meadii, Byssoloma xanthonicum stands out due to its pale hypothecium and is more similar in colour and appearance to B. leucoblepharum.