The existence of a region of low pH juxtaposed to the proximal jejunum has been proposed to explain anomalies in the transfer of weakly-ionizing substances: yet no direct evidence for this layer exists. Surface electrode studies on everted sacs revealed the existence of the layer and estimated the pH to be at least 5.5 when the buffer pH is 7.2. With 60 $\mu $m long tip microelectrodes, a value of 6.0 was obtained, however, the dimensions of this microclimate are uncertain. Optical experiments with pH-indicators using a microscope of 20 $\mu $m resolving power and flat strips in vitro, failed to detect this layer. With the dimensions of the tip microelectrode known, values are given for the maximum possible microclimate pH for a corresponding microclimate depth. The significance of the pH microclimate to malabsorption is briefly discussed.