The present invention relates to a glass composition which is resistant to alkalis and to acids, in particular for the preparation of reinforcing glass strands, which comprises the following constituents within the limits defined below, as percentages by weight: TABLE-US-00001 SiO.sub.2 .gtoreq.58%, preferably .ltoreq.65% ZrO.sub.2 15-20% R.sub.2O (R = Na, K or Li) .gtoreq.14% K.sub.2O .ltoreq.0.1%, preferably .ltoreq.0.05% RO (R = Mg, Ca or Sr) 2.5-6% MgO .ltoreq.4% TiO.sub.2 >1 and .ltoreq.4% the composition additionally being devoid of F, comprising less than 1% of impurities (Al.sub.2O.sub.3, Fe.sub.2O.sub.3 and Cr.sub.2O.sub.3) and satisfying the following relationships: ZrO.sub.2+TiO.sub.2.gtoreq.17% ZrO.sub.2/TiO.sub.2.gtoreq.6 It also relates to the use of the glass strands obtained in the reinforcing of inorganic materials, for example cementitious materials, or organic materials, for example plastics, and to the composites including such strands.