Method for filling a catalytic reactor comprising a plurality of tubular reaction compartments having an inlet and a closed end, the method comprising the following steps: a). placing a layer of a first catalyst in the closed end of the tubular reaction compartments; b) placing a layer of a second catalyst in the tubular reaction compartments on top of the layer of first catalyst; the first catalyst and the second catalyst being granular catalysts; in which: - step a) involves inserting first inserts into the tubular reaction compartments then introducing the first catalyst into the first inserts, each first insert comprising: • a tubular body designed to be introduced into a tubular reaction compartment, • a head designed to collaborate with the inlet of the tubular reaction compartment in order to hold the first insert in place, • the head being provided with a longitudinal through-passage communicating with the inside of the tubular body; - step b) involves inserting second inserts into the tubular reaction compartments then introducing the second catalyst into the second inserts, each second insert comprising: • a tubular body designed to be introduced into a tubular reaction compartment, • a head designed to collaborate with the inlet of the tubular reaction compartment in order to hold the second insert in place, • the head being provided with a longitudinal through-passage communicating with the inside of the tubular body; - the tubular bodies of the first inserts having a longitudinal dimension greater than the longitudinal dimension of the tubular bodies of the second inserts; - the heads of the first inserts being visually distinct from the heads of the second inserts; and - the first inserts being removed before the second inserts are inserted. a). placing a layer of a first catalyst in the closed end of the tubular reaction compartments; b) placing a layer of a second catalyst in the tubular reaction compartments on top of the layer of first catalyst; - step a) involves inserting first inserts into the tubular reaction compartments then introducing the first catalyst into the first inserts, each first insert comprising: • a tubular body designed to be introduced into a tubular reaction compartment, • a head designed to collaborate with the inlet of the tubular reaction compartment in order to hold the first insert in place, • the head being provided with a longitudinal through-passage communicating with the inside of the tubular body; • a tubular body designed to be introduced into a tubular reaction compartment, • a head designed to collaborate with the inlet of the tubular reaction compartment in order to hold the first insert in place, • the head being provided with a longitudinal through-passage communicating with the inside of the tubular body; - step b) involves inserting second inserts into the tubular reaction compartments then introducing the second catalyst into the second inserts, each second insert comprising: • a tubular body designed to be introduced into a tubular reaction compartment, • a head designed to collaborate with the inlet of the tubular reaction compartment in order to hold the second insert in place, • the head being provided with a longitudinal through-passage communicating with the inside of the tubular body; • a tubular body designed to be introduced into a tubular reaction compartment, • a head designed to collaborate with the inlet of the tubular reaction compartment in order to hold the second insert in place, • the head being provided with a longitudinal through-passage communicating with the inside of the tubular body; - the tubular bodies of the first inserts having a longitudinal dimension greater than the longitudinal dimension of the tubular bodies of the second inserts; - the heads of the first inserts being visually distinct from the heads of the second inserts; and - the first inserts being removed before the second inserts are inserted.