Zero field (ZF) and transverse field (TF) muon spin relaxation and rotation (μ SR) experiments have been carried out in the Cd-doped heavy fermion superconductor CeCoIn5 to investigate its superconducting state. The ZF-μ SR results in CeCo (In0.97 Cd0.03)5 revealed that no spontaneous magnetic field was induced below its superconducting transition temperature (Tc), indicating no evidence for time reversal symmetry breaking. The muon Knight shifts obtained from TF-μ SR measurements decrease significantly below Tc, consistent with a spin-singlet state as in the parent compound CeCoIn5, which is a d-wave superconductor. © 2008 Elsevier B.V.