The mean observed pH of a 5.00 plus or minus 0.05 x 0.00001 M H2504 solution was 4.06 plus or minus 0.05 (2s) for 485 pH measurements by seven different operators, using nine Orion Ross Model 81-94b pH combination electrodes and four different pH meters over 8 weeks. Traditional methods using activity coefficients give a pH of 4.005 for this solution and the liquid junction potential error, yields a calculated pH for this standard of 4.030. The difference between the calculated pH, corrected by a larger residual liquid junction potential error than was calculated for this electrode with the currently available methods. Consequently, the assumption of a negligible residual junction potential error in future pH measurements of dilute acid standards is subject to question in accurate work. Further progress in the field seems limited by the non-availability of technological innovations which provide highly reproducible liquid junction potential errors or minimize temperature equilibration errors in pH probes. (Copyright (c) Analyst, November 1987, Vol. 112.)