CO and NO geminate recombination to ferrous horseradish peroxidase (HRP) was investigated using picosecond laser pulses. In free HRP-CO, approximately 15 - 20 % geminate recombination was observed with a first-order rate constant k = (7 ± 1) x 10⁸ s^-1. In the presence of substrates [Ph(C=O)NH-X, X = OH, H, NH₂, and CH₃] the extent of geminate recombination increased to 70 - 90 %, with a k ranging from 1 to 3 x 10⁹ s^-1. These results imply that steric effects constrain the substrate which traps more CO inside the heme cavity. NO geminate recombination appeared to be biphasic, with the faster phase occurring within the laser pulse (approx. 30 ps). However, the range of k values for the slower process was found to be 1 - 3 x 10¹⁰ s^-1, which is 10-fold faster than CO geminate recombination. Comparison of CO and NO geminate recombination in HRP and myoglobin suggests different mechanisms of control of ligand binding within their heme cavities.