Semi-aerobic landfilling is applied increasingly as a sustainable technology worldwide, although frequently controversial results are achieved. The authors suggest that differences in water availability (climate, moisture content, etc.) and putrescible waste content are the key factors involved in controlling performance and efficiencies. The aim of the present study was to investigate the effect of inverse conditions (high/low) of these two factors. Six lab-scale lysimeters were specifically set up to correspond to three different conditions of water availability (wet conditions, dry conditions and artificially controlled watering under dry conditions) and two different waste types (high and low putrescible content). Lysimeters were operated for four months under thermal-insulated conditions and the quality and quantity of emissions monitored regularly. Concentrations of mobile ammonia and total organic carbon (TOC) in landfilled waste were modelled by means of first-order kinetics, and carbon and nitrogen mass balances were calculated. The best performance for the semi-aerobic process was achieved at a water availability of approximately 1.5-2.4 kgH2O/kgTS using the following two combinations: a) Waste with high putrescible content and no addition of external water due to the presence of sufficient endogenous water in the waste (moisture) to promote biological stabilisation of waste (Respiration index in 4 days, RI4 = 12.87 mgO2/gTS, BOD/COD < 0.05); b) Waste with low putrescible content and controlled watering (RI4 = 12.25 mgO2/gTS, BOD/COD < 0.04). The study highlighted how semi-aerobic landfilling operations should be carefully adjusted case by case according to waste quality and climate conditions.