The reduction of odours is still the major challenge for the swine industry. Several additives on the market claim to reduce odours. But how can the producer be sure of their efficiency? It was necessary to develop an evaluation protocol of these products that would be acknowledged by the scientific community and the concerned organisms. First, a standardized method for producing pig slurry was developed. Tunnels were then designed to simulate manure spreading conditions in the field. The minimum required number of eight panelists, carefully selected and trained, performed the olfactory evaluation. The experimental design ensured a sufficient number of replications for each treatment, within reasonable cost limits. Four additives were tested: two slurry additives, one biological and the other chemical (to be incorporated in the temporary storage pit), a feed additive (to be incorporated in the diet) and finally a combination of a feed additive and a slurry additive. To be significant, the difference between the control and each treatment had to be equal or superior to a percentage for every parameter (for example, a difference of at least 25% between odour concentrations). Secondly, the statistical confidence interval had to be 95% or more for every parameter. Those criteria allowed for the variabilities due to the different measurement methodologies. According to our study, none of these four additives showed a significant impact. Producers are encouraged to check with their supplier if the product they are using has been tested with this protocol and, if so, to see the results.