A method for dosing reagents in water disinfection plants, wherein said method comprises: a) Providing a plant which is a circuit containing water, wherein said circuit is cyclically open or closed; b) Providing a disinfectant substance D, wherein said disinfectant substance D is monochloramine or chlorine dioxide and said disinfectant substance D is obtained by a reaction of the type A + B -> D, or A + B + C -> D, or A + B -> C + D and the same disinfectant substance D undergoes a degradation reaction of the type D -> A + B + S 1 +... + S n or D + B -> A + S 1 +... + S n; c) Introducing said substance D, or reagents A + B, or A + B + C into the water circulating in said plant in a suitable stoichiometric ratio where said reaction of forming the disinfectant substance D takes place in the circulating water; d) Periodically monitoring, directly and/or indirectly, the concentration levels of said reagent or degradation product A and/or of said reagent or degradation products B and/or of said disinfectant substance D in the circulating water and comparing said measured levels, defined as A x, B x, D x, with the reference threshold levels, defined as A MAX, B MAX, D EFF, wherein said monitoring takes place approximately at 9:00 PM, at 10:00 PM, at 11:00 PM, at 00:00 AM, at 3:00 AM, at 7:00 AM, at 8:00 AM, at 12:00 PM; e) Periodically introducing said reagent/degradation product A and/or B in circulation in an amount of (α ± ε) *V and/or (β ± ε)*V, respectively, or said disinfectant substance D in an amount of (τ + ε)*V, where V is the total volume contained in the closed circuit, α, β, τ indicate the amount of A, B, D to be added, as determined in advance as a function of the following parameters: plant volume and plant usage frequency and characteristics of the fresh water introduced therein; ε is a correction factor determined after said comparison of A x to A MAX and/or B x to B MAX and/or D x to D EFF, wherein said correction factor ε modifies from 5% to 10% said amounts α, β, τ. a) Providing a plant which is a circuit containing water, wherein said circuit is cyclically open or closed; b) Providing a disinfectant substance D, wherein said disinfectant substance D is monochloramine or chlorine dioxide and said disinfectant substance D is obtained by a reaction of the type A + B -> D, or A + B + C -> D, or A + B -> C + D and the same disinfectant substance D undergoes a degradation reaction of the type D -> A + B + S 1 +... + S n or D + B -> A + S 1 +... + S n; c) Introducing said substance D, or reagents A + B, or A + B + C into the water circulating in said plant in a suitable stoichiometric ratio where said reaction of forming the disinfectant substance D takes place in the circulating water; d) Periodically monitoring, directly and/or indirectly, the concentration levels of said reagent or degradation product A and/or of said reagent or degradation products B and/or of said disinfectant substance D in the circulating water and comparing said measured levels, defined as A x, B x, D x, with the reference threshold levels, defined as A MAX, B MAX, D EFF, wherein said monitoring takes place approximately at 9:00 PM, at 10:00 PM, at 11:00 PM, at 00:00 AM, at 3:00 AM, at 7:00 AM, at 8:00 AM, at 12:00 PM; e) Periodically introducing said reagent/degradation product A and/or B in circulation in an amount of (α ± ε) *V and/or (β ± ε)*V, respectively, or said disinfectant substance D in an amount of (τ + ε)*V, where V is the total volume contained in the closed circuit, α, β, τ indicate the amount of A, B, D to be added, as determined in advance as a function of the following parameters: plant volume and plant usage frequency and characteristics of the fresh water introduced therein; ε is a correction factor determined after said comparison of A x to A MAX and/or B x to B MAX and/or D x to D EFF, wherein said correction factor ε modifies from 5% to 10% said amounts α, β, τ.