The purpose of this study was to assess the validity and reliability of a Cosmed K4b 2 portable telemetric gas analysis system. Twelve physically fit males performed a treadmill running session consisting of an easy 10 min run, a hard 3 min run and a 1 min sprint (with rest periods of 10 min separating each run), on four separate occasions. Sessions were identical with the exception of the apparatus used to measure V̇O 2 . During two (test-retest) sessions a Cosmed K4b 2 portable gas analysis system was used; in another, a laboratory metabolic cart and, in one session, both systems were used to measure V̇O 2 simultaneously. Comparison of Cosmed K4b 2 and metabolic cart measurements in isolation revealed significantly (p<0.05) increased values of V̇O 2 , V̇CO 2 , F E CO 2 (except F E CO 2 at 10min) and lower values of F E O 2 for each run duration by the Cosmed system. Linear regression equations to predict metabolic cart results from Cosmed values were, respectively; cart V̇O 2 = 0.926 ( Cosmed V̇O 2 − 0.227 (r 2 = 0.84) and cart V̇CO 2 = 1.057 ( Cosmed V̇CO 2 − 0.606 (r 2 = 0.92). Bland-Altman plots and comparison of the test-retest Cosmed measurements revealed that the K4b 2 system showed good repeatability of measurement for measures of V̇ E , V̇O 2 and V̇CO 2 , particularly for 10 min and 3 min tests (ICC = 0.7 – 0.9, p<0.05). In conclusion, the Cosmed K4b 2 portable gas analysis system recorded consistently higher V̇O 2 and V̇CO 2 measurements in comparison to a metabolic cart. However, satisfactory test-retest reliability of the system was demonstrated.