AbstractHyperfine structure, Stark effect and Zeeman effect of the TlCl molecule have been measured with a molecular beam apparatus using electric four poles as deflecting fields and a homogeneous electric field parallel to a superimposed magnetic field in the transition region. Electric dipole transitions were induced between the hyperfine structure levels of the first rotational state J = 1 in both strong and weak external field.The following quantities could be evaluated from the spectra: the electric dipole moment µel and the magnetic rotational dipole moment µJ of the molecule, the nuclear spin-rotational interactions c1 and c2, the scalar and tensor part of the nuclear dipole-dipole interaction dS and dT, the quadrupole coupling constant e q Q of the Cl nucleus, the anisotropy of the magnetic susceptibility ξ⊥− ξ∥ , the anisotropy of the magnetic shielding of the external magnetic field at the position of both nuclei (σ⊥- σ∥)1 and (σ⊥- σ∥)2, the magnetic moment of the Cl nucleus multiplied by the scalar part of the magnetic shielding tensor µ2 · (1 - σS)2. For the most abundant isotop 205Tl35Cl the vibrational dependence of most of these quantities was measured in the vibrational states v =0, 1, 2, 3. Isotopic effects for 203Tl35Cl, 205Tl37Cl and 203Tl37Cl were investigated in the ground vibrational state. In addition the vibrational dependence of the electric dipole moment was measured for all isotopic species.It is pointed out that the usual connections between (σ⊥- σ∥)1,2 and c1,2 and between ξ⊥− ξ∥ and µJ do not hold when the excited electronic states of the molecule obey Hund’s coupling case c, which occurs most probably in TlCl.