Abstract: The problem. Bowen hypothesized that substance abusers typically have low levels of differentiation, are often cutoff from their families to avoid high level of emotional anxiety and intense involvement they may experience in the relationships. In this study of 256 subjects, the differentiation level in terms of (1) Emotional Reactivity, (2) I Position, (3) Emotional Cutoff, and (4) Fusion with Others of 128 adult individuals in an inpatient chemical dependent treatment program was compared to that of 128 non-dependent adults at a local Christian church to determine if there was a significant difference between the two groups. Median age of first use for alcohol was determined to be 13, and 15 for drugs in the test sample. The differentiation level of chemical dependent individuals who started using alcohol at 13 or 'Younger' was also compared to that of individuals who began to abuse alcohol at 14 and 'Older.' Similar comparison study was also conducted between chemical dependent individuals who began to use drugs at 15 and 'Younger' versus 16 and 'Older.' A correlation of strong family life events was also examined between individuals who began to use alcohol and drugs at a younger age, versus those who started abusing at an older age. Method. The test and control samples completed a list of questionnaires adapted from Skowron & Friedlander (1998) to measure the differentiation variables being studied. Mean scores on the four differentiation scales and total differentiation of the test versus control samples were compared using t-tests. The total alcohol and drugs users were divided at their respective age-of-first-use median into two groups: 'Younger' and 'Older.' Mean scores on the differentiation scales for the 'Younger' versus 'Older' groups were compared using t-tests. Chi Square was used to compare 'Younger' and 'Older' groups in terms of experiencing or not experiencing the different family life change events. Results. Data from this study indicated significant lower differentiation level among adult chemical dependent individuals than that in the control population. The chemical dependent sample also scored higher than did the non-dependent population on Emotional Reactivity, Emotional Cutoff, Fusion with Others, and lower in I Position. These findings supported the suppositions of Bowen Family Systems theory and also revealed that people who began using alcohol at age 13 and younger had significantly higher Emotional Reactivity scores than did those who started using alcohol at age 14 and older. More of the 'Younger' alcohol users were sent to live with other relatives, reported family financial hardship and had family members with legal problems than did 'Older' users. The data further indicated significantly more people who started using drugs at age 15 or younger had family members that used alcohol, reported family arguments over alcohol and drugs, were sent to live with another relative, had family financial hardship, reported increased parental arguments and had family members with legal problems than did 'Older' abusers.