The design of simple nonlinear observer-based estimators for the on-line estimation of the reactions＇ rates in chemical reactors is presented. It is also shown that these estimates can be used for the on-line estimation of the unmeasured components＇ concentrations. The proposed estimators do not assume or require any model for the reactions＇ kinetics and are very successful in accurately estimating these variables. They are presented in both continuous-and discrete-time versions. A main characteristic of these estimators lies in the easiness of their implementation and in particular their calibration. Indeed, their tuning is reduced to the calibration of a single parameter and this is true regardless of the number of reactions＇ rates to be estimated. The performances of the proposed estimators are highlighted through experimental results.