Patients with heart failure with preserved ejection fraction (HFpEF) exhibit cardiopulmonary abnormalities that could affect the predictability of exercise [Formula: see text] from the Jones corrected partial pressure of end-tidal CO (PJ) equation (PJ = 5.5 + 0.9 × [Formula: see text] - 2.1 × V). Since the dead space to tidal volume (V/V) calculation also includes [Formula: see text] measurements, estimates of V/V from PJ may also be affected. Because using noninvasive estimates of [Formula: see text] and V/V could save patient discomfort, time, and cost, we examined whether partial pressure of end-tidal CO ([Formula: see text]) and PJ can be used to estimate [Formula: see text] and V/V in 13 patients with HFpEF. [Formula: see text] was measured from expired gases measured simultaneously with radial arterial blood gases at rest, constant-load (20 W), and peak exercise. V/V was calculated using the Enghoff modification of the Bohr equation, and estimates of V/V were calculated using [Formula: see text] (V/V) and PJ (V/V) in place of [Formula: see text]. [Formula: see text] was similar to [Formula: see text] at rest (-1.46 ± 2.63, = 0.112) and peak exercise (0.66 ± 2.56, = 0.392), but overestimated [Formula: see text] at 20 W (-2.09 ± 2.55, = 0.020). PJ was similar to [Formula: see text] at rest (-1.29 ± 2.57, = 0.119) and 20 W (-1.06 ± 2.29, = 0.154), but underestimated [Formula: see text] at peak exercise (1.90 ± 2.13, = 0.009). V/V was similar to V/V at rest (-0.01 ± 0.03, = 0.127) and peak exercise (0.01 ± 0.04, = 0.210), but overestimated V/V at 20 W (-0.02 ± 0.03, = 0.025). Although V/V was similar to V/V at rest (-0.01 ± 0.03, = 0.156) and 20 W (-0.01 ± 0.03, = 0.133), V/V underestimated V/V at peak exercise (0.03 ± 0.04, = 0.013). Exercise [Formula: see text] and V/V provides better estimates of [Formula: see text] and V/V than PJ and V/V does at peak exercise. Thus, estimates of [Formula: see text] and V/V should only be used if sampling arterial blood during CPET is not feasible. [Formula: see text] provides a better estimate of [Formula: see text] than PJ at peak exercise, and V/V provides a better estimate of V/V than V/V at peak exercise. Although we reported significant correlations, we did not find an identity between [Formula: see text] and estimates of [Formula: see text], nor did we find an identity between V/V and estimates of V/V. Thus, caution should be taken and estimates of [Formula: see text] and V/V should only be used if sampling arterial blood during CPET is not feasible.