Formaldehyde (HCHO) is a principal intermediate in the photochemical oxidation of hydrocarbons in the troposphere. Isotopic analysis is an important tool for tracing the atmospheric path of gaseous species, and for this purpose, characterization of the isotope effects in the loss processes for formaldehyde is needed. The inain loss pathways for formaldehyde in the atmosphere are photolysis and reactions with the radical species of OH, Cl, Br, and NO3. In this study, the kinetic isotope effects in the reactions of five different isotopomers of formaldehyde (HCHO) with OH, Cl, Br, and NO3 radicals are studied in a relative-rate experiment at 298 +/- 2 K and 1013 +/- 10 mbar. The reaction rates of DCDO, HCDO, H-13 CHO, and (HCHO)-O-18 with the four radicals are measured relative to H2CO in a smog chamber using long-path FTIR detection. The experimental data are analyzed with a nonlinear least-squares spectral -fitting method using measured high-resolution infrared spectra and cross sections from the HITRAN database. The reaction rates of HCDO and HCH(1)8O with OH and Cl are determined relative to HCHO as k(OH+HCHo)/k(OH+HCDO) = 1.28 +/- 0.01, k(OH+HCHO)/k(OH+HCH)(O)(18) = 0.967 +/- 0.006, k(Cl+HCHO)/k(Cl+HCDO) = 1.201 +/- 0.002, and k(Cl+HCHO)/k(Cl+HCH)(O)(18) 1.08 +/- 0.01. The reaction rates of HCDO and (HCHO)-O-18 with Br and NO3 are determined relative to HCHO as k(Br+HCHO)/k(Br+HCDO) = 3.27 +/- 0.03, k(Br+ HCHO)/k(Br+HCH)(O)(18) = 1.275 +/- 0.008, k(NO3+HCHO)/k(NO3+HCDO) 1.78 +/- 0.0 1, and k(NO3+HCHO)/k(NO3+HCH)(O)(18) 0.98 +/- 0.01. The errors represent 2sigma from the statistical analyses, and do not include possible systematic errors.