Here in we describe our findings about the behaviour of some N-aryl-and N-heteroaryl-N'-alkylthioureas towards electrophilic reagents. In acid medium, the treatment of thioureas bearing aryl groups with 4-chloropyridine in 2-propanol yielded N-aryl-N-(4-pyridyl)-N'-alkylthioureas and N-aryl-N'-alkylureas, whereas the heteroarylthioureas tested under similar reaction conditions afforded N-heteroaryl-N'-alkyl-O-(2-propyl)isoureas. The reaction of N- (5,6,7,8-tetrahydronaphth-1-yl)-and N-(2-benzimidazolyl)-N'-butylthiourea with propargyl bromide in acid medium led to the formation of 2-butylimino-3-arylthiazolines, in a regioselective way. However, when this reaction was carried out in basic conditions the regioselectivity failed and a mixture of isomeric thiazolines was obtained. The Z-or E-configuration of the imino group of the synthesized thiazolines was studied by molecular modelling and by selective nuclear Overhauser experiments in nuclear magnetic resonance.