IEC 60793-1-50:2014 provides a practical method for evaluating fibre performance in a defined environment. The purpose of this standard is to determine the suitability of optical fibre sub-category A1a to A1d multimode fibres and class B and C single-mode fibres to withstand the environmental condition of high humidity and high temperature which may occur in actual use, storage and/or transport. The test is primarily intended to permit the observation of effects of high humidity at constant temperature over a given period. This procedure is conducted in accordance with IEC 60068-2-78, Test Cab. This second edition cancels and replaces the first edition, published in 2001, and constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - harmonization of the content with sectional specifications of relevant fibre types; - extension of the applicability of the standard to class C single-mode fibres. Keywords: evaluating fibre performance, sub-category A1a to A1d multimode fibres, class B and C single-mode fibres, high humidity and high temperature