Heart failure with preserved ejection fraction (HFpEF) patients exhibit cardiopulmonary abnormalities that could affect the predictability of exercise PaCO2 from the Jones (PJCO2) equation (PJCO2=5.5+0.9xPETCO2-2.1xVT). Since the dead space to tidal volume (VD/VT) calculation also includes PaCO2 measurements, estimates of VD/VT from PJCO2 may also be affected. Because using noninvasive estimates of PaCO2 and VD/VT could save patient discomfort, time, and cost, we examined whether PETCO2 and PJCO2 can be used to estimate PaCO2 and VD/VT in 13 HFpEF patients. PETCO2 was measured from expired gases measured simultaneously with radial arterial blood gases at rest, constant-load (20W), and peak exercise. VD/VT[art] was calculated using the Enghoff modification of the Bohr equation, and estimates of VD/VT were calculated using PETCO2 (VD/VT[ET]) and PJCO2 (VD/VT[J]) in place of PaCO2. PETCO2 was similar to PaCO2 at rest (-1.46±2.63, P=0.112) and peak exercise (0.66±2.56, P=0.392), but overestimated PaCO2 at 20W (-2.09±2.55, P=0.020). PJCO2 was similar to PaCO2 at rest (-1.29±2.57, P=0.119) and 20W (-1.06±2.29, P=0.154); but, underestimated PaCO2 at peak exercise (1.90±2.13, P=0.009). VD/VT[ET] was similar to VD/VT[art] at rest (-0.01±0.03, P=0.127) and peak exercise (0.01±0.04, P=0.210), but overestimated VD/VT[art] at 20W (-0.02±0.03, P=0.025). Although VD/VT[J] was similar to VD/VT[art] at rest (-0.01±0.03, P=0.156) and 20W (-0.01±0.03, P=0.133), VD/VT[J] underestimated VD/VT[art] at peak exercise (0.03±0.04, P=0.013). Exercise PETCO2 and VD/VT[ET] provide better estimates of PaCO2 and VD/VT[art] than PJCO2 and VD/VT[J] does at peak exercise. Thus, estimates of PaCO2 and VD/VT should only be used if sampling arterial blood during CPET is not feasible.