The rotational Zeeman effect in the J → J′ = 0 → 1 and 1 → 2 rotational transitions has been observed for H-C≡C-C≡ 14 N, D-C≡C-C≡ 14 N, D-C≡C-C= 15 N, and H-C≡C-C= I5 N using a microwave bridge superheterodyne spectrometer. From the experimental g ⊥ -values, susceptibility anisotropies and rotational constants, the molecular electric quadrunole moment, which plays an important role for collisional excitation and relaxation, follows as Q ∥ =2.14(9) x 10 -26 esu cm 2 (referred to the center of mass of the H-C≡C-C≡ 14 N species). Since the Deuterium quadrupole hyperfine splittings in the Zeeman spectra could be resolved, it was also possible to Fit the Deuterium quadrupole coupling constant. Our value, eqQ ( 2 H) = 198.2(46) kHz is about 10% smaller than the value derived from a recent high resolution microwave Fourier transform study. The reasons for this discrepancy are discussed. All experimental molecular parameters are compared to the results of ab initio quantum chemical calculations and it is argued that the 14 N nuclear electric quadrupole moment is possibly about 30% larger than presently assumed.