The objective of this study consisted in developing a theory about the psychotherapeutic process in Psychoanalytic Psychotherapy and to verify the relevance of the common and specific factors. the participants were three psychoanalytical psychotherapists. Semi-structured interviews and 15 session's notes given by one of the therapists (one psychotherapeutic process of a 24 year old psychotic patient) were used and analysed according to the Grounded Theory method. A process called constant modification was found in the core of psychoanalytic psychotherapy, through which changes occur in the psychic functioning and in the relational style of the patient. This process functions at an intrapsychic and extrapsychic level. A model based on the concept of Enzymatic Catalysis was developed, in order to integrate and relate common factors (therapeutic bond), specific factors (therapeutic interventions), patient characteristics, therapist characteristics, external factors and therapeutic results to the central process. It was concluded that both common and specific factors contribute to the therapeutic process in a mutual dynamic.