The following information is from the metalink website:
Limitations. ------------
The following limitations apply to the EXPORT and IMPORT utilities (see details in the matrix below):
1. Oracle export dump files can only be read by the Oracle IMPORT utility because these dump files are stored in a special Oracle-binary format.
2. Every export dump file is importable into all future major, patch, and maintenance releases of Oracle.
3. The export dump files cannot be read by IMPORT utilities of previous maintenance releases and versions. So, a version 9 export dump file cannot be imported by a version 8i IMPORT utility (possible errors: IMP-10 and IMP-21), and a version 8 export dump file cannot be imported by a version 7 IMPORT utility (possible errors: IMP-69 and IMP-21). In both cases, the import will terminate with the error IMP-0 "Import terminated unsuccessfully".
4. IMPORT can read export dump files created by EXPORT release 5.1.22 and higher (up to same version).
5. IMPORT cannot read export dump files created by the EXPORT utility of a higher maintenance releases or versions. So, a release 8.1 export dump file cannot be imported by a release 8.0 IMPORT utility, and a version 8 export dump file cannot be imported by a version 7 IMPORT utility.
6. The Oracle6 (or earlier) EXPORT utility cannot be used against an Oracle8 or Oracle8i or higher release database.
7. Whenever a lower version EXPORT utility runs with a higher version of the Oracle Server, any categories of database objects that did not exist in the lower version are excluded from the export. For example, partitioned tables are not exported by the version 7 EXPORT utility. If you need to move a version 8 partitioned table to a version 7 database, then first reorganize the table into a non-partitioned table.
Examples.
---------
1. From 7.3.3 to 8.1.6 => Use the EXPORT 7.3.3 to export the data from the
7.3.3 database and IMPORT 8.1.6 to import the data
into the 8.1.6 database.
2. From 8.1.7 to 8.1.7 => Use the EXPORT 8.1.7 to export the data from the
8.1.7 database and IMPORT 8.1.7 to import the data
into the 8.1.7 database.
3. From 9.0.1 to 8.1.7 => Use the EXPORT 8.1.7 to export the data from the
9.0.1 database and IMPORT 8.1.7 to import the data
into the 8.1.7 database.
4. From 8.1.7 to 9.2.0 => Use the EXPORT 8.1.7 to export the data from the
8.1.7 database and IMPORT 9.2.0 to import the data
into the 9.2.0 database.
5. From 9.0.1 to 9.2.0 => Use the EXPORT 9.0.1 to export the data from the
9.0.1 database and IMPORT 9.2.0 to import the data
into the 9.2.0 database.
6. From 8.1.7 to 7.3.4 => Only if never run before, first run CATEXP7.SQL
on the 8.1.7 database in order to create the
Oracle7 data dictionary views; then use the
EXPORT 7.3.4 to export the data from the 8.1.7
database and IMPORT 7.3.4 to import the data into
the 7.3.4 database.
7. From 9.0.1 to 7.3.4 => Not supported.
8. From 8.0.5 to 9.0.1 => Not supported with export FULL=Y and import FULL=Y
9. From 8.1.5 to 9.2.0 => Not supported with export FULL=Y and import FULL=Y