Molecular diagnostic ratios (MDRs)—the ratios of defined pairs of individual compounds—have been widely used as markers of different source categories of polycyclic aromatic hydrocarbons (PAHs). However, it is well-known that variations in combustion conditions and environmental degradation processes can cause substantial variability in the emission and degradation of individual compounds, potentially undermining the application of MDRs as reliable source apportionment tools. The United Kingdom produces a national inventory of atmospheric emissions of PAHs, and has an ambient air monitoring program at a range of rural, semirural, urban, and industrial sites. The inventory and the monitoring data are available over the past 20 years (1990–2010), a time frame that has seen known changes in combustion type and source. Here we assess 5 MDRs that have been used in the literature as source markers. We examine the spatial and temporal variability in the ratios and consider whether they are responsive to known differences in source strength and types between sites (on rural–urban gradients) and to underlying changes in national emissions since 1990. We conclude that the use of these 5 MDRs produces contradictory results and that they do not respond to known differences (in time and space) in atmospheric emission sources. For example, at a site near a motorway and far from other evident emission sources, the use of MDRs suggests “non-traffic” emissions. The ANT/(ANT + PHE) ratio is strongly seasonal at some sites; it is the most susceptible MDR to atmospheric processes, so these results illustrate how weathering in the environment will undermine the effectiveness of MDRs as markers of source(s). We conclude that PAH MDRs can exhibit spatial and temporal differences, but they are not valid markers of known differences in source categories and type. Atmospheric sources of PAHs in the UK are probably not dominated by any single clear and strong source type, so the mixture of PAHs in air is quickly “blended” away from the influence of the few major point sources which exist and further weathered in the environment by atmospheric reactions and selective loss processes.