© 2014. The American Astronomical Society. All rights reserved. We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic center for methyl formate, HCOOCH 3 , and its isotopologues H 13 COOCH 3 and HCOO 13 CH 3 . The observations were carried out with the APEX telescope in the frequency range 283.4-287.4 GHz. Based on the APEX observations, we report tentative detections of the 13 C-methyl formate isotopologue HCOO 13 CH 3 toward the following four massive star-forming regions: Sgr B2(N-LMH), NGC 6334 IRS 1, W51 e2, and G19.61-0.23. In addition, we have used the 1 mm ALMA science verification observations of Orion-KL and confirm the detection of the 13 C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the 12 C/ 13 C isotope ratio in methyl formate toward the Orion-KL Compact Ridge and Hot Core-SW components (68.4 ± 10.1 and 71.4 ± 7.8, respectively) are, for both the 13 C-methyl formate isotopologues, commensurate with the average 12 C/ 13 C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the 12 C/ 13 C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H 13 COOCH 3 and HCOO 13 CH 3 species. New spectroscopic data for both isotopomers H 13 COOCH 3 and HCOO 13 CH 3 , presented in this study, have made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.