Purpose. The purpose of this investigation was to determine the effect of body movement on the ability of a portable telemetric gas analysis device, the Cosmed K4b2, to produce a reliable measure of walking oxygen consumption (VO2). Methods. Thirteen adults were asked to report to a laboratory for three test sessions. During Session 1, participants were familiarized with the data collection procedures and completed a 15-min treadmill accommodation. In Sessions 2 and 3, each participant completed four 5-min treadmill walking trials at 1.8 m · sec-1 under different experimental conditions while VO2 was collected using the portable system. The four conditions were used to manipulate the portable metabolic system to mimic different types of movement. Results. Data analysis revealed no significant differences (p = 0.070) amongst VO2 values across all four experimental conditions during Sessions 2 and 3. Between-day coefficients of variation for the mean VO2 values of the four conditions ranged from 2.7% to 3.8%, with the highest level of variation occurring whilst the unit was strapped to the participant’s chest. Conclusions. Viewed collectively, the Cosmed K4b2 may produce reliable measures of VO2 in adult participants during treadmill walking who exhibit little vertical oscillation. However, future research is necessary to determine if the K4b2 is a valid instrument for collecting VO2 data if excessive body movement in the vertical direction can affect the data collection instrumentation.