The following new species of Arthoniales are described from Rondônia: Alyxoria fuscospora with 3-septate clavate ascospores of 20–23×4·5–5·5 µm having a gelatinous layer soon appearing evenly brown (also known from other tropical countries). Chiodecton complexum with discrete soralia, immersed apothecia in branched lines and 3-septate ascospores (26–)33–40×2·5–3·5 µm. Coniarthonia rosea, similar to C. pulcherrima but with the apothecia pink and more irregular in outline and the ascospores 13–16×5·5–6·5 µm. Cresponea flavosorediata with yellow-olive soralia, apothecia with yellow pruina and 7–9-septate ascospores, 26–38(–50)×5·0–6·5 µm. Cresponea lichenicola, lichenicolous on a Pyrenula, with 0·1–0·3 mm wide apothecia with yellow pruina. Eremothecella helicella with helicoid curved c. 17–29-septate conidia c. 70–95×2 µm long if uncoiled, with septa 3–6 µm apart. The Arthoniales are a speciose component of the lowland rainforest of Rondônia. A key is provided to the corticolous species of most groups of Arthoniales found in Rondônia. The foliicolous species found so far are listed. Most are new records for Rondônia. Chrysothrix occidentalis is new to the Neotropics.

Chrysothrix occidentalis Elix & Kantvilas, C. palaeophila Kantvilas & Elix and C. tchupalensis Elix & Kantvilas are described as new to science. In addition, the new combination Chrysothrix sulphurella (Räsänen) Kantvilas & Elix is proposed, C. granulosa G. Thor is reported for the first time from Australia (including Tasmania) and New Zealand, and C. xanthina (Vain.) Kalb for the first time from Tasmania, Norfolk Island and New Zealand. A key to the Australian species of Chrysothrix is presented. Chemical data on non-Australasian species of the genus are also provided.