The paper presents CFD results for the transonic flow of dry and moist air through a diffuser and a compressor rotor.In both test geometries,i.e.the Sajben transonic diffuser and the NASA Rotor 37,the air humidity impact on the structure of flows with weak shock waves was examined.The CFD simulations were performed by means of an in-house CFD code,which was the RANS-based modelling approach to compressible flow solutions.It is shown that at high values of relative humidity,above 70％,the modelling of the transonic flow field with weak shock waves by means of the dry air model may produce wrong results.