The grounds for disqualification of arbitrators provided by most arbitration rules refer to the relationships of arbitrators with the parties or the subject matter of the dispute – i.e., issue conflicts. However, some of those rules used to be vague or unclear, which resulted in incomplete arbitrators’ disclosures, unfunded challenge requests and awards that lacked homogeneity. Following a case-study research method, the paper focuses on the ICSID investor-State dispute settlement mechanism. It analyses and compares the ICSID Convention and Arbitration Rules, the UNCITRAL Arbitration Rules and the IBA Guidelines on Conflicts of Interest in light of challenge awards. The paper stresses the relevance of arbitrators’ disclosure duty and the importance of clearly defining the scope and content of ‘impartiality’ and ‘independence’ as a guidance for parties and deciding authorities when raising or deciding upon a challenge request. In particular, the study aims to show how international investment arbitrations deal with issue conflicts as grounds for disqualification, by analyzing their main causes: multiple appointments of arbitrators, similar legal issues to be decided and permission to serve both as counsel and arbitrator.