The concentration of diatomic hydrogen in the liquid phase of an anaerobic digester was used to determine the onset of digester failure induced by substrate overloading. The construction of an inexpensive probe to measure dissolved hydrogen, having a partial pressure detection limit of 30 Pa, is described. An increase in the partial pressure of dissolved hydrogen, from less than 30 Pa to 400 Pa, was observed when the D-glucose concentration in a laboratory-scaled digester was increased rapidly to 10 mM. However, when the digester was gradually overloaded, an increase in the dissolved-hydrogen partial pressure was not observed until after the digester failed. The accumulation of volatile fatty acids and digester failure were observed at dissolved-hydrogen partial pressures below 30 Pa.