Abstract Microwave spectra of the complete set of monosubstituted isotopic pyrazoles have been investigated and the complete r s structure of pyrazole determined. The deuterated and 15 N species of pyrazole were prepared, while the 13 C species were examined in natural abundance. The spectrum of the parent compound was remeasured yielding rotational and centrifugal distortion constants. The mutual position of the hydrogen atoms and the direction of the dipole moment in the pyrazole principal axis system has been unambiguously established. The 14 N quadrupole hyperfine structure was resolved and the quadrupole coupling constants derived by Blackman et al. [8] were confirmed. An estimated r z structure is compared to crystal structures derived by X-ray and neutron diffraction showing the N-H bond in the crystal to be prolonged. But, otherwise no significant differences seem to develop when bringing the molecule from its isolated state into the crystal with its strong hydrogen bonds.