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The rotational spectra of the isotopomers mecp···H79Br, mecp···H81Br and mecp···D79Br (mecp=methylenecyclopropane) of a complex formed between mecp and hydrogen bromide were observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Accurate values of the rotational constants A0, B0 and C0, the centrifugal distortion constants ΔJ, ΔJK and δJ, and the components χaa, (χbb-χcc) and χab of the Br nuclear quadrupole coupling tensor were determined in each case. These spectroscopic constants were interpreted to show that mecp···HBr has Cs symmetry, with the HBr subunit lying in the principal inertial plane ab, which coincides with the molecular symmetry plane. The H atom of HBr forms a hydrogen bond to the centre (*) of the mecp π bond. The angle ϕ included by the unique C atom of the cyclopropyl ring, the centre of the π bond and the H atom of HBr is 88.8(10)° and the distance r(*···H)=2.353(18) Å. The hydrogen bond *···H–Br deviates from linearity by ϑ=18.03(23)° in a direction that suggests a secondary interaction of δ-Br with two of the protons of the cyclopropyl ring. The properties ϕ, ϑ, r(Z···X) and r(Hring···Y) of the complexes (CH2)2Z···XY, where Z=O, S or C2CH2 and XY=HCl, HBr or ClF are compared and the reason why the hydrogen bonds Z···H–Y are significantly nonlinear, while the ‘chlorine’ bonds Z···Cl–F remain close to linearity, is considered.