We have studied the temperature dependence of the lattice parameters and the in uence of spin anisotropy on the electron paramagnetic spectra of Cu(tn)Cl₂, an S = 1=2 quasi-two-dimensional spatially-anisotropic triangularlattice Heisenberg antiferromagnet. The variation of the resonance fields with temperature reflects the presence of an easy-plane exchange anisotropy with J_z=J_x,y < 1 and g-factor anisotropy, g_z=g_x,y > 1.