Soil respiration is tightly linked to rhizosphere processes, which are characterized by high spatial variability. We tested whether planar optodes can be applied to capture this spatial variability of CO2 in the rhizosphere. Maize (Zea mays) was grown in rhizoboxes and CO2 concentration around roots was measured at three volumetric soil water contents (VWC) using planar optodes. Gradients of CO2 were clearly visible around root tips but less pronounced around mature root parts probably due to high root respiration and microbial activity around tips. Saturating the soil from 21% VWC increased the measured CO2 concentration from 0.23 to 1.47 μmol CO2 L-1. Statistical comparisons between VWC levels indicated that small changes in soil WC might strongly affect CO2 measurements. However, provided that the soil moisture is kept constant, optode measurements of CO2 can be used in moist soil samples to quantify relative differences in CO2 concentration between treatments or soil regions.