The use of automation has increased in all the fields of the pharmaceutical industry, such as manufacturing and quality control. In the field of in vitro dissolution testing, the fiber-optic system is often considered a promising tool because it allows in situ monitoring of the drug release without manual sampling, liquid transfer, and hence phase separation. This paper evaluates the use of fiber-optic in vitro dissolution testing and presents a technique to compensate for analytical interferences caused by excipients. INTRODUCTION To assess product performance and the quality of pharmaceuticals, in vitro dissolution tests are regularly carried out in the pharmaceutical industry. Over the past decades, these tests were more commonly performed manually. The tendency in industry is to reduce random error and optimize time and cost by automation. Dissolution testing is a composite procedure, with the bath as a sample preparation device, sampling and other transfer actions for liquids, filtration to obtain the portion dissolved, chemical analysis (HPLC, spectrophotometry), and data processing at the end. All of the steps may introduce additional error, decreasing the precision of the final results. Automated systems may include automated sampling, processing of sample solutions, online quantification, and even cleaning. However, to reduce the number of steps to both accelerate the process and increase the precision of dissolution methods, alternative solutions may allow omission of the sample and filtration steps by measuring drug release directly in the vessels. Suitable probes that are connected by glass fiber to the spectrophotometer enable UV measurements at any selected time point. The goal of this paper is to review the suitability of fiber-optic systems for dissolution testing and to describe the limitations for use. An example of a fiber-optic system with a “rod” probe is provided to expand the use. A method to compensate for the interferences by undissolved particles during in situ monitoring of the drug release is presented. DESCRIPTION OF THE SYSTEM USED FOR THE EXPERIMENTS The system comprised a UV–vis spectrophotometer (Varian Cary 50 Tablet) equipped with the Cary WinUV software and connected to an Agilent dissolution apparatus 708–DS by fiber optics. Among the different types of probes (1), the rod type was selected (Figure 1). The complete fiber-optic system is commercially available from Agilent Technologies. *Corresponding author. e-mail: johannes.kraemer@phast.de Figure 1. Temperature probe (left) with optic rod probe (right) (source: Agilent Technologies). dx.doi.org/10.14227/DT200213P15