Fourier transform microwave spectra of HCSCN and HC³⁴SCN showing the 3_0, 3–2_0, 2 rotational transition. The spectra show the hyperfine components, labelled with the corresponding values of quantum numbers F′–F′′, clearly resolved. The coaxial arrangement of the adiabatic expansion and the resonator axis produces an instrumental Doppler doubling. The resonance frequencies are calculated as the average of the two Doppler components.