This study shows an expert confirmed initial list of factors which influence the complexity of the integration process of ERP systems and non ERP systems. After a thorough search for complexity factors in scientific literature, a survey amongst 8 experts in a leading European long special steel products company, which was recently composed out of independent international companies, was conducted. The participants confirmed the retrieved list from literature, consisting of 5 quantitative and 21 qualitative factors. The participants added one extra qualitative factor and scored the importance of all factors. Three quantitative factors, i.e. a technology, a business and a project factor, scored highest. When dealing with integration issues, this initial list of factors can provide awareness for organizations to support activities such as planning, control and risk management