Hello. The thickest paper we have tested on the ET-5850 is Epson Ultra Premium Photo Paper Glossy, which weighs 79 lb (approx 297 gsm). We have also tested Epson Ultra Premium Presentation Paper Matte, which is similar to cardstock and works with the machine. However, that paper weighs around 44 lb (approx 165 gsm). You can always try cardstock that falls in that range at your discretion, but please note that your results can vary when using 3rd party paper, as these recommendations are for Epson paper only. Regards, Kevin - The Epson Team.