Inclusion Criteria:  Already using overnight CPAP and tolerating the therapy well (over 4 hours use/night) Hypercapnia: venous bicarbonate of 30 milliequivalents of solute per liter (Eq/L) or more, OR transcutaneous carbon dioxide (TcCO2) of 50 millimeters of mercury (mmHg) of more. Provide written informed consent Already use a full (oronasal) face mask for CPAP Willing to have a venous blood sample taken during the pre-screening visit.  Exclusion Criteria:  Presence of sleep apnea on CPAP:  Subjects excluded if they have an apnea-hypopnea index (AHI) of more than 20, or If the AHI has not already decreased by 50 percent or more from pre-CPAP to with CPAP. Forced expiratory volume in 1 second (FEV1) of less than 40 percent of predicted.  Forced expiratory volume in 1 second divided by forced vital capacity (FEV1/FVC) less than fifty percent of predicted, at the screening visit.  Have poorly controlled asthma or changing asthma symptoms. Nasal obstruction Do not fit Mask A or Mask B Agitation or psychological illness