The Mexico City Metropolitan Area (MCMA) produces a complex mixture of gases and aerosols from diverse sources, including burning of fossil fuels, biomass, and wastes, with a significant biogenic contribution. We present the first results of ongoing projects to study temporal and spatial variations of 14CO2 in the area. Temporal variations reconstructed from tree rings of Taxodium mucronatum indicate a considerable radiocarbon depletion, in accordance to the vast amount of fossil fuels burnt inside Mexico Valley, with values between 62 and 246‰ lower than background values for the 1962–1968 period, and lower by 51–88‰ for the 1983–2010 period. The lower dilution found for the last decades might indicate an increase in enriched 14CO2 sources. Results from the spatial distribution, as revealed from integrated CO2 samples and grasses from six points within the MCMA collected during the 2013 dry season, show variations between sites and sample types. For integrated CO2 samples, values range from 35.6‰ to 54.0‰, and for grasses between −86.8‰ and 40.7‰. For three of the sampling points, the grasses are significantly depleted, by up to ∼133‰, as compared to the corresponding integrated CO2 sample. This may result from differences in the carbon assimilation period and exposure to different CO2 sources. Higher-than-background Δ14C values were found for all integrated CO2 samples, presumably resulting from 14C-enriched CO2 derived from forest fires in the mountains during the sampling period. Results obtained so far confirm the complexity of the 14C cycle in the MCMA.