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Multi-cone diffusers have good air distribution characteristics in cooling applications. However, surface condensation has been a problem impeding the use of multi-cone diffusers in cold air applications. An experimental study and a computer analysis are presented to show the causes of surface condensation on a multi-cone diffuser when distributing cold air. This study also attempts to alter the flow field characteristics near the diffuser cone surfaces by using an open inner cone with an insert that has a porous structure. Computer analysis has shown that the resultant airflow from the inner cone will repel the entrained room air away from the diffuser, thus preventing contact of the more humid air with the cold cone surfaces. The analysis has also shown that the altered flow field will cause the air dew point temperature in the vicinity of the diffuser to be lower than the temperature of the cone surfaces. Inserts with different porosities that were effective in preventing surface condensation were tested and the results are presented. These results indicate that the proposed airflow control method is effective in preventing surface condensation.