To compare pH and PCO2 values of simultaneously obtained arterial, arterialized capillary, and venous blood samples and also to compare oxygen saturation (ASaO2) measured in arterial blood and oxygen saturation by pulse oximetry (PSaO2).Prospective study was done in the children admitted in the Pediatric Intensive Care Unit of Christian Medical College Hospital Vellore, requiring critical care. All the three blood gas samples (arterial, capillary and venous) were taken simultaneously and analyzed. Oxygen saturation by pulse oximetry was also recorded.50 children aged 14 days to 12 years were included in the study. Arterial and capillary pH values were highly correlated (r2=0.9024, p<0.0001). Out of 16 children with arterial acidosis 9(56%) were identified by capillary blood gas. Arterial and venous pH values also showed good correlation (r2=0.8449, p<0.0001). The PCO2 values of arterial and capillary blood gases were found to be highly correlated (r2=0.9534, p<0.0001). The capillary blood gas accurately reflected the arterial PCO2 in 41 (82%) patients. Arterial and venous blood gas PCO2 values had less correlation (r2=0.5917, p=0.011). The arterial oxygen saturation (ASaO2) and oxygen saturation by pulse oximetry (PSaO2) were correlated moderately (r2=0.7241, p<0.0001).Even though arterial blood gas analysis is the gold standard, and when an arterial blood gas sample cannot be obtained, a combination of arterialized capillary blood gas and pulse oximetry can be effectively used in acutely ill children of all ages. Venous samples have a good correlation with arterial samples for pH but are not useful for monitoring blood gas status in acutely ill children.