Background: Timing and types of complementary feeding in infancy affect nutritional status and health later in life. The present study aimed to investigate the factors associated with early introduction of complementary feeding (i.e., before age 4 months), and factors associated with infants consumption of non-recommended foods, including sweet beverages and snack foods. Methods: This study used cross-sectional data from the BeeBOFT study (n = 2157). Data on complementary feeding practices and potential determinants were obtained by questionnaire at infant's age of 6 months. Logistic regression models were used to investigate factors associated with early introduction of complementary feeding and infants' consumption of non-recommended foods. Results: 21.4% of infants had received complementary feeding before 4 months of age. At the age of 6 months, 20.2% of all infants were consuming sweet beverages daily and 16.5% were consuming snack foods daily. Younger maternal age, lower maternal educational level, absence or shorter duration of breastfeeding, parental conviction that "my child always wants to eat when he/she sees someone eating" and not attending day-care were independently associated with both early introduction of complementary feeding and the consumption of non-recommended foods. Higher maternal pre-pregnancy BMI and infant postnatal weight gain were associated only with early introduction of complementary feeding. Conclusions: We identified several demographical, biological, behavioral, psychosocial, and social factors associated with inappropriate complementary feeding practices. These findings are relevant for designing intervention programs aimed at educating parents. Trial registration: The trail is registered at Netherlands Trial Register, trail registration number: NTR1831. Retrospectively registered on May 29, 2009.