Abstract: A thermowell for a fixed-bed reactor which has a vertical spine portion, and multiple spider portions spaced along the spine portion. At least one thermocouple and preferable a plurality of thermocouples are located in each arm of the spider portions with thermocouple leads extending upwardly through the spine portion to a monitor outside of the reactor. The thermowell is constructed in the reactor while loading the catalyst by first positioning the spine portion in the reactor, and then fabricating a spider portion at the lowermost level of the spine portion with at least one thermocouple in each arm of the spider portion and thermocouple leads extending through the arms and upwardly through the spine portion. Catalyst is then added to the reactor to cover the spider portion at the lowermost level. At each successive level, repeating the steps of fabricating another spider portion, and of adding catalyst to cover the spider portion.