Abstract. Term weighting is one of the most important aspects of modern Web retrieval systems. The weight associated with a given term in a document shows the importance of the term for the document, i.e. its usefulness for distinguishing documents in a document collection. In search engines operating in a dynamic environment such as the Internet, where many documents are deleted from and added to the database, the usual formula involving the inverse document frequency is too costly to be computed each time the document collection is updated. This paper proposes two new simple and effective weighting functions. These weighting functions have been tested and compared with results obtained for the PIVOT, SMART and INQUERY methods using the WT10g collection of documents.