Radiation treatment with catheter-based β-emitter sources is currently under clinical trial to prevent restenosis. In the present paper, we address the characterization of the high-dose-rate ⁹⁰Sr–⁹⁰Y seeds of the Beta-Cath™ system supplied by Novoste Corporation, one of the commercially available sources for intravascular brachytherapy. The Monte Carlo code PENELOPE has been used to simulate the transport of electrons emitted by the encapsulated ⁹⁰Sr–⁹⁰Y seeds. The calculated radial dose function and anisotropy function for a single seed in water are compared with simulation results from other authors. Regarding g(r), the present result lies between the ITS3 and EGS4 curves, being somewhat closer to ITS3, while in the case of F(r, θ) some differences appear for certain angular intervals and radial distances. In order to put the observed differences into perspective, we have calculated radial doses for point isotropic sources in water. Our results for 0.5 and 1 MeV electrons are in good agreement with simulations using EGSnrc, and an excellent agreement is obtained with ITS for point ⁹⁰Sr–⁹⁰Y emitters. Dose distributions in water are calculated for source 'trains' consisting of 1, 2, 3, 4, 5, 9 and 12 seeds. The dose at the source midplane is enhanced if the number of seeds is up to 4, and saturates for trains with 5 or more seeds. We also compare the dose distribution obtained by simply adding the contributions of individual seeds with the simulation of the complete source train. It is found that both calculation procedures yield essentially the same result for distances greater than about 2 mm. Finally, the contribution of bremsstrahlung photons to the dose is briefly analysed.