We present simultaneous fast, in-situ measurements of formaldehyde and glyoxal from two rural campaigns, BEARPEX 2009 and BEACHON-ROCS, both located in Pinus Ponderosa forests with emissions dominated by biogenic volatile organic compounds (VOCs). Despite considerable variability in the formaldehyde and glyoxal concentrations, the ratio of glyoxal to formaldehyde, <i>R</i><sub>GF</sub>, displayed a very regular diurnal cycle over nearly 2 weeks of measurements. The only deviations in <i>R</i><sub>GF</sub> were toward higher values and were the result of a biomass burning event during BEARPEX 2009 and very fresh anthropogenic influence during BEACHON-ROCS. Other rapid changes in glyoxal and formaldehyde concentrations have hardly any affect on <i>R</i><sub>GF</sub> and could reflect transitions between low and high NO regimes. The trend of increased <i>R</i><sub>GF</sub> from both anthropogenic reactive VOC mixtures and biomass burning compared to biogenic reactive VOC mixtures is robust due to the short timescales over which the observed changes in <i>R</i><sub>GF</sub> occurred. Satellite retrievals, which suggest higher <i>R</i><sub>GF</sub> for biogenic areas, are in contrast to our observed trends. It remains important to address this discrepancy, especially in view of the importance of satellite retrievals and in-situ measurements for model comparison. In addition, we propose that <i>R</i><sub>GF</sub>, together with the absolute concentrations of glyoxal and formaldehyde, represents a useful metric for biogenic or anthropogenic reactive VOC mixtures. In particular, <i>R</i><sub>GF</sub> yields information about not simply the VOCs in an airmass, but the VOC processing that directly couples ozone and secondary organic aerosol production.