We are carrying out a study of the physical parameters and excitation of low-ionization structures (LIS) in planetary nebulae (PNe). Since the optical morphology and kinematics of the LIS and the main components (rims, shells and haloes) of our sample were studied previously, our main goal now is to search for: i) the density contrasts between jets/knots and the main nebular components; ii) their main excitation processes; and iii) their chemical abundances. The first results of this survey --- based on the analysis of NGC 7009, NGC 6543, NGC 6891 and K 4-47 --- are that there is no significant density contrast between LIS and their surroundings, and that most of the LIS studied (but not all) are mainly photoionized, rather than shock excited.