If you are looking at the reaction of a single compound, the addition to the system an another potentially degradable compound would affect the main reaction. This is a well known problem especially in the case of co-oxidation involving radicals. There is no general answer to your question. A co-oxidation might result in whatever imaginable. Radical scavengers themselves could affect on the processes of degradation. The simplest way to minimize the effects of co-oxidation is to use the so called method of initial rates. This means that you measure only the very beginning of the process, where you have a minimum number of reactants. Ignoring the co-oxidation (co-reactions) will definitely result in a wrong interpretation of data.

ould the early period of the reaction be before the t½? In this time region, the rate constant will be different from that of the whole process. A simpler approach is to use the initial reaction rate, as I saw in a paper where they calculated the contribution using the following equation:

S.CONTRIBUTION=-rRc-prod/rpolluant

(−rpollutant​) is the disappearance rate of the pollutant in initla stage , and rRC-prodr​ is the formation rate of the organic product that comes from the direct reaction of each specific reactive species (RC) with the pollutant. Both rates are evaluated at the initial stage of the reaction. However, this equation was used after simulating the reaction chain, not from experimental data.

As I wrote, there is no general answer to your question. The initial reaction rate is commonly calculated at conversions less than 10%. If you want to quantitavely describe the effect of co-oxidation, you should build the kinetic model and simulate the process. This is an extremely comlicated task.