• Using a new disposable polymer sensor, tissue pH was continuously monitored successfully in 33 critically ill neonates. Tissue pH correlated well with the arterial pH at wide pH ranges [n = 476; r=.84; y = 0.46 +(0.94)x] Tissue pH changes accurately reflected the arterial pH during both respiratory and metabolic acidosis and alkalosis. During severe metabolic acidosis (arterial pH <7.10) tissue pH was lower than arterial pH. No complications were encountered during or after monitoring. Compared with earlier studies using a glass electrode, this electrode reflects a marked technologic improvement because of its ease of application, longevity of use, disposability, and decreased risk of complications in small preterm infants. Unlike transcutaneous oxygen and carbon dioxide sensors, this device is invasive but is not heated and does not require frequent site changes.