The molecular structure for acetylenemethyldioxorhenium, (C2H2)(CH3)ReO2, was obtained by measuring and analyzing the rotational spectra for 14 isotopomers. This appears to be the first gas-phase measurement for a rhenium metallacyclopropene, and the first structural data on this compound. This complex is closely related to reaction intermediates in methyltrioxorhenium and osmium tetroxide catalyzed oxidation reactions, which are important in industrial chemical production and syntheses of chiral products. The microwave spectra were measured in the 4-11 GHz range using a Flygare-Balle-type pulsed-beam microwave spectrometer. The structural parameters obtained using the 42 measured rotational constants are in very good agreement with results from the new density functional theory calculations. The structure of the acetylene ligand is modified through interaction with the metal atom, and exhibits partial sp(2) hybridization in the complex. The C-C bond length is increased by 0.09 Angstrom to 1.29 Angstrom. The H-C-C interbond angles are reduced from 180 degrees to 146 degrees, and 147 degrees. The measured rhenium, methyl-carbon bond length is 2.116(2) Angstrom, and rhenium-acetylene carbon bond lengths are 2.043(2) Angstrom and 2.067(2) Angstrom. Large quadrupole splitting patterns arising from the two rhenium isotopes [chi (cc)(Re-185)=812.62 MHz, and chi (cc)(Re-187)=769.023 MHz], with large off-diagonal components [\chi (ab)(Re-185)\=725.40 MHz, and (\chi (ab)((187) Re)\=686.384 MHz] caused some initial difficulty in assigning transitions. When the assignments and analysis were completed, the systematic changes in the measured quadrupole coupling tensor components due to rotations of the principal inertial axes on isotopic substitution were obtained, and helped to confirm assignments for the various isotopomers.