In this letter we characterize the candidates for young stars and brown dwarfs detected in a survey with ISOCAM in the Chamaeleon II dark cloud. Photometric data coming from a wide-field optical imaging survey, combined with IR measurements from the 2MASS catalogue and from the Spitzer satellite, allow us to study the nature of the candidates. Using intermediate-band filters we can provide a first estimate of the temperatures for objects cooler than about 3800 K. On the basis of spectroscopic and photometric data, we confirm that ISO-CHA II 13 is a young substellar object with a mass of about 0.05~M&sun;. It is thus the first object in Cha II whose substellar nature has been spectroscopically confirmed. By having a temperature of about 2880 K and displaying a substantial infrared excess, it joins the list of young brown dwarfs observed to have a surrounding disk.