The following simple empirical correlation was developed to relate the thermowell temperature, T-tw, in tubes containing concentric axial thermowells to the centerline and mean temperatures, T-a and T-m, in tubes without thermowells at the same axial location in packed bed multitubular reactors : (T-a-T-tw)/(T-a-T-m)=36.51(d(tw)/d(t))(1.9)-(34.05-3.955/Bi)(d(tw)/d(t))(2) 0 less than or equal to d(tw)/d(t) less than or equal to 0.3, 1 less than or equal to Bi less than or equal to 10. In conjunction with a reactor model of the tubes without thermowells, use of this correlation provides an easy, accurate method of predicting the thermowell temperature for comparison with data. This correlation was developed from computer simulations of pseudo-homogeneous, plug flow, packed bed tubular reactors with and without thermowells. T-tw is always lower than T-a for exothermic reactions; thus, stable operation in the instrumented tubes does not guarantee that the tubes without thermowells will not experience runaway. At industrial operating conditions, thermowell sheath conduction has a negligible effect on the temperature measurement of the surrounding bed.