The present invention relates to a chemically resistant glass composition for the production of reinforcing strands which comprises the following constituents within the limits defined below, expressed in mol %: SiO.sub.2 67-72%; ZrO.sub.2 5-9.5%, preferably .gtoreq.7.5%; R.sub.2O (R=Na, K and Li) 11-17%; Li.sub.2O 0-5.5%; K.sub.2O 0-5.5%; Na.sub.2O<10%; and CaO 3-9%, the composition furthermore containing less 1% of impurities (Al.sub.2O.sub.3, Fe.sub.2O.sub.3, Cr.sub.2O.sub.3, TiO.sub.2, MgO, SrO, BaO and P.sub.2O.sub.5) and being free of F. It also relates to the glass strands obtained from this composition and to the composites based on an organic or inorganic material containing such strands.