A composition based on a polypropylene homopolymer and/or copolymer for preparing caps for carbonated drinks has enhanced properties of rigidity and leak-tightness without the need for using additional seals. The composition includes: 85.65 to 97.87 parts of a polypropylene homopolymer and/or copolymer; 1.5 to 12 parts of an ethylene-octene and/or ethylene-methyl acrylate elastomer, and an additive selected from 0.3 to 1.2 parts of a lubricating and/or sliding agent; 0.05 to 0.3 parts of a nucleation agent; 0.08 to 0.25 parts of an antioxidant and/or antistatic agent, and 0.2 to 0.6 parts of an antistatic agent. The polypropylene polymer is selected from a polypropylene homopolymer and has an isotacticity greater than 94% and a MFR of 0.5-7 g/10 min, and/or the polypropylene copolymer is selected from a heterophasic copolymer (ethylene-propylene) and has an isotacticity greater than 88% and a MFR of 0.5-7 g/10 min and/or mixtures of same.