The superconducting properties of CaTSi3 (where T = Pt and Ir) have been investigated using muon spectroscopy. Our muon-spin-relaxation results suggest that in both these superconductors time-reversal symmetry is preserved, while muon-spin-rotation data show that the temperature dependence of the superfluid density is consistent with an isotropic s-wave gap. The magnetic penetration depths determined from our transverse-field muon-spin-rotation spectra are found to be 448(6) and 150(7) nm for CaPtSi3 and CaIrSi3, respectively. © 2014 American Physical Society.