Drying is a complex thermal process and a crucial stage in the manufacture of many products in various industries. Reliable and accurate process monitoring techniques are needed to achieve a consistent product quality. A new and innovative method was developed that uses passive acoustics as a non-invasive, real-time monitoring technique for end-point detection in rotary dryers. This study used molecular sieves as an example of solid particles for which tight control of product moisture is crucial. A higher than ideal moisture results in sieves with a poor adsorption performance while a lower moisture results in excessive attrition. Current end-point detection methods available for this critical manufacturing stage are inadequate and prevent the production of molecular sieves with the best possible adsorption capability. A microphone recorded acoustic emissions during the drying of molecular sieves in a 1.5 m diameter rotary dryer. Signal analysis revealed that the critical end-point, corresponding to maximum moisture removal without attrition, can be detected by standard deviation analysis of the acoustic emissions. This provides a useful monitoring technique that can prevent the significant losses associated with over-drying. Multi-linear regression analysis of the acoustic emissions using wavelets or power spectral density provides an effective method of achieving a specific moisture content. Passive acoustic monitoring for end-point determination can be easily implemented into current industrial drying processes to prevent over-drying, reduce production costs, and improve product quality and consistency. (c) 2006 Elsevier B.V. All rights reserved.