Contamination on outdoor insulators enhances the chances of flashover and can lead to transmission line outages and reduced system reliability. In practice, various contaminant types, usually classified as soluble and insoluble, settle on outdoor insulators and increase the possibility of flashover. The paper presents an assessment of the flashover performance of such insulators in Egypt; in view of the nature and composition of contaminants accumulated on their surfaces. To quantify these contaminants, numerous samples are gathered from different regions of the country and subjected to a comprehensive chemical analysis at the scale analysis laboratory of the Egyptian Electricity Transmission Company. Measurements of equivalent salt deposition density (ESDD) surface conductivity (SC), maximum leakage current (MLC) and flashover voltage (FOV) were conducted and correlated to the contamination constituents. The study utilized IEC standard cap-pin suspension insulators which were hanged in regions of different pollution levels and nature. Regression models for predicting the flashover voltage as related to ESDD and MLC as related to surface conductivity were developed. The models are validated through statistical analysis as well as comparison with measured data. The results from this work are useful for utilities to take adequate measures to improve the efficiency of their transmission systems through scheduling of their insulator maintenance strategy and selecting the appropriate insulator design for different system voltages and regions.