•Oxidation of beryllium is enhanced by neutron irradiation and elevated humidity.•Elevated humidity and neutron irradiation lower thermal process temperatures.•Oxidation of beryllium yields a mobile needle- and cluster-like oxide layer.•Tritium inclusions due to neutron irradiation are released above 1040 K.•Beryllium oxide structure depends on the humidity level in airflow. This study focuses on neutron-irradiated beryllium interactions with air of differing humidity, simulating water cooled fusion reactor malfunction. Beryllium in the form of ∼ 1 mm ∅ pebbles has been used in the study. Both non-irradiated and neutron-irradiated pebbles were thermally treated and studied using thermogravimetric/differential thermal analysis in dry airflow with relative humidity (RH) < 5 % and humid > 90% RH. Surface structure and composition was investigated with scanning electron microscopy (SEM) and energy dispersion X-ray spectrometry (EDX). Tritium release was monitored using a tritium monitor. Results show that elevated RH coupled with neutron irradiation damage on beryllium yields the largest mass increase of 180% (in relation to the initial mass) compared to non-irradiated Be treated in RH < 5% which resulted in 3 % mass increase. SEM and EDX revealed a substantial number of cracks as well as the formation of irregular shape micro-/nanoparticles of BeO. Tritium release took place at temperatures above ∼ 1040 K, reaching a peak after 1263 K. Overall tritium inventory was estimated to be up to 1.08∙1017 T atoms/g.